When God said, "Reason is also choice", he meant that a person can choose either reasonably or unreasonably on one hand, but on the other, it will be the latter's uninfluenced choice, and no one else's. Adam and Eve, as well as Satan, show the truth behind the divine statement, as all of them show relatively willingly the freedom to make their choice. However, while they chose the wrong one, they knew it, and thus do not have this excuse to offer God.
Now, to Milton and his like-minded contemporaries, God was the omnipotent and omnipresent presence in their life, the Creator. God created everything that was right and good, including the freedom of will and reason to chose, and to abuse these gifts for one's selfish ends was simply wrong and foolish - the sign of hubris, in fact. Consequently, Satan appears in "Paradise Lost" as a character with extremely proud temper. He rebels against God because he becomes unsatisfied with his position, and chose to seek a way to alter it.
Satan and his crew are the first to choose not to submit to divine authority. Starting from his first appearance in Book I, Satan is a master orator; he already has a plan to make himself look innocent, he always has a prepared version of reality. When the fallen angels begin to recover already in Hell, Satan claims:
If then his providence Out of our evil seek to bring forth good, Our labour must be to pervert that end, And out of good still to find means of evil; Which ofttimes may succeed so as perhaps Shall grieve him, if I fail not, and disturb His inmost counsels from their destined aim.
(Paradise Lost, I. 157-168)
In the first two books of the poem, Satan acts unrepentant of his actions; he became an adversary to God seemingly without any second thoughts or regrets. However, the initial audience of the poem saw Satan's choice of actions and his reasoning as incorrect from the start - God created everything, including Satan, and to turn on Him would be a bad, unreasonable choice. True, God is supposed to be forgiving of one's mistakes and misconceptions, but only if you are ready to admit them. Satan's state of mental affairs is nowhere near this admittance at this point.
Then Satan arrives in Eden and sees for himself all that he has forsaken in favor of Hell; when he is finally alone and without an audience to put a show for, Satan speaks some very different lines - the truth:
Me miserable! which way shall I fly Infinite wrath, and infinite despair? Which way I fly is Hell; myself am Hell; And, in the lowest deep, a lower deep Still threatening to devour me opens wide, To which the Hell I suffer seems a Heaven. O, then, at last relent: Is there no place Left for repentance, none for pardon left? None left but by submission; and that word Disdain forbids me, and my dread of shame Among the Spirits beneath, whom I seduced With other promises and other vaunts Than to submit, boasting I could subdue The Omnipotent.
(III, 83-86)
As Satan willingly chose to forsake God, despite all of his posturing, he became miserable, even more so than his underlings, for they do not comprehend as fully their loss of the divine miracle. Yet despite his understanding of the miracle, Satan is determined to stay and remain evil and impenitent. He knows that that is what he will always choose of his own volition - his reason has the flaw of pride, and unless he abandons it, nothing will ever change.
Whereas Satan descended because of his overblown opinions regarding self-worth, arrogance and pride, Adam and Eve also fell because of arrogance. Man has both free will and reason to resist the temptation of Satan, who is determined to destroy them because of his inherent wickedness, and so when he submits to Satan instead, it is his choice alone, and is no part of God's. Moreover, the actions of Eve and Adam in book IX prove this state of events true.
In book IV, before the Fall, Adam and Eve is a perfect couple as both humans and spouses. This comes natural to them, because they are perfect, God created them, and they have not yet sinned. Thus, they are perfect, not just physically (to Satan in book IV (pg. 878, lines 358-365), they appear to possess god-like physiques), but mentally as well: their relationship is of mutual respect and co-operation, there is nothing lowly or carnal about it. Unlike Satan, Adam and Eve understand that God is greater than they are because He is, and chose to accept this state of affairs without the complaint or anguish of Satan's choice.
Yet, already there are signs that Eve has a personality flaw, her physical appearance. When she sees her reflection for the first time in the river, she just stares at it because it is so good-looking, although it is clearly unreasonable (pg 879, 460-465). Furthermore, when God tells her to go to Adam, she almost does not do so, because he is less good-looking than she is (pg. 879, 477-480), and divine intervention is needed to rein her in. Eve acts not due to her reason, but to her emotions, which cloud her ability to choose and decide. In book IX, this flaw causes Eve and Adam's downfall.
The breakdown begins with Eve leaving Adam to seek their enemy by herself. Satan, however, catches her with words of flattery, which Eve chooses to accept, despite Adam's warning:
For he who tempts, though in vain, at least asperses
The tempted with dishonour foul; supposed
Not incorruptible of faith, not proof
Against temptation
(IX, 297-301)
Nonetheless, Eve's physical vanity and the resulting pride cloud her power to reason things out. Satan understands this, and so he lies.
Amid the Tree now got, where plenty hung Tempting so nigh, to pluck and eat my fill I spar'd not, for such pleasure till that hour At Feed or Fountain never had I found. Sated at length, ere long I might perceave Strange alteration in me, to degree Of Reason in my inward Powers, and Speech Wanted not long, though to this shape retain'd.
(IX. 595-603)
Satan purposefully and clearly lies about himself and his self: book IX firmly closes the question of Satan's nature: he is a liar and a fraud; nothing but Sin and Death could come from him.
Yet, Eve believes those lies and goes with Satan to the forbidden tree. In book IV (pg 879, 426-430), it is revealed that the tree is forbidden because it is a statement of God's power that mankind had only what God gave it, nothing more. Adam knew it and told Eve; yet Eve, when she sees the tree, rather than tell Satan the truth, chooses to tell that if she and Adam ate the tree's fruit, they would die. Satan works with this false statement to falsely prove to Eve that if she and Adam ate the tree's fruit she and Adam would become equal with God - but they simply could not be equal to God, and Adam knew that too, and Eve probably as well. Ergo, her taking and eating the forbidden fruit means that she chose to overstep her bounds and reject the divine order. Eve chose to act unreasonable by believing a snake over a divine decree in order to promote herself. Thus, when she got cast out of Eden, and became less perfect in body, less godly, so to say, it is because she chose this to be her fate, when she accepted the snake and its speeches at face value, rather than think them through and remember the truth of the matter.
Adam, however, immediately understands what Eve actually did, yet when he learns what Eve does, he is aghast. Yet, Adam chose to share the sins of Eve, even though he is still pure and clean in spirit and reason, unlike her.
God left free the Will; for what obeys
Reason, is free; and Reason He made right,
But bid her well beware, and still erect;
Lest, by some fair-appearing good surprised,
She dictate false; and mis-inform the will
To do what God expressly has forbid.
(IX, 353-358)
By accepting Eve's proposal, however, Adam acts contrary to reason and will, which suggested that God is always the right choice over lesser attractions: "One Flesh" he proclaims himself with Eve, and thus they end up sharing the same affliction: their understanding of the world, their reason, became clouded. That is further reflected when they have sex with each other: base lust and carnal desire entered their hearts, and made them even more imperfect - moreover, this conscious, chosen descent from mental perfection and reason culminates in their foolish attempt to hide from God in fear.
In the conclusion of the poem, there is nothing but reason and conscious choice, as far as Milton is concerned. Consciously chosen sins of all three have resulted in their diminishment and removal from God. Satan and his rebel angels are turned into snakes, while Adam, Eve, and their descendants are denied physical immortality forever, proving the statement of the Old Testament - "an eye for an eye, a tooth for a tooth": if you chose to do the unreasonable and defy God, he punishes you in return - it is simple, straightforward, and proves Milton's statement that reason is choice, even if that reason is not of our day and age.
Bibliography:
Paradise Lost.Black, Joseph, et al., eds. The Broadview Anthology of British Literature. The Renaissance and the Early Seventeenth Century. 1st ed. 6 vols. Petersborough: Broadview Press, 2002. Pg. 856, 874, 889, 892, 896