The Theon in the novel starts off as very unreflective while being a moral coward. We do not see what follows between the first and second chapter -- it's likely he did think, a little, but he goes where the wind blows, taking the path of least resistance. It's only once he's buried in the mire of his own betrayal that the cracks show and you realize that this was all a horrible, petty mistake that happens because of that moral cowardice of his.
The thing is, GRRM can easily do this -- he can mask that part of the story, and only bring it out later while we're in his head and we start seeing the regret and shame seep in through the cracks. They can't put us directly into Theon's head, however, in the same way... hence this wonderful scene, which externalizes the struggle where it most makes sense.
It's a medium thing. They take different approaches -- they front-load it, GRRM puts it at the tail end -- but the result is the same: fantastically complex characters.
Of course, sometimes they front load too damned much...
I agree that in the later books, after enduring unspeakable torture, Theon/Reek begins to express a small measure of regret, sorrow, perhaps even a little repentance. And he does show a modicum of loyalty by keeping secret the fact that Bran and Rickon are alive. But is there any textual evidence that suggests that he was actually conflicted about betraying Robb and the Starks initially? My recollection is that it was a pretty easy decision for him. He is upset to discover that his father does not trust him and that he has been usurped by his sister; but are there any indications that he feels any pains of conscience when he allies himself with his father's plans? Perhaps there are. It's easy to forget or overlook these kinds of details in the books. My impression of Theon is that he is driven by ambition, lust, and a desire to prove himself to his father. Breaking vows and bonds of affection and loyalty seem to come easy to him. I cannot see book-Theon ever composing a letter of warning to Robb. It is only later, after his education at the hands of Ramsay Bolton, that he begins to evidence something akin to remorse. At least that is my recollection. Perhaps I'm wrong.