The Lion Turtle told Aang that his spirit had to be pure, completely and utterly pure, in order to take away (hot) Ozai's bending, so how is Amon doing it? A few things I must point out:
1) When Aang took Ozai's bending, there was light coming from all openings in the body (well, at least in the face openings) but when Amon does it, the bending simply flickers out into nothingness (like in the episode. There was powerful lightning that flickered into a lesser power, then into fire, then a small flame then... nothing.) So what's up with that?
2) Aang touched Ozai's chest and forehead. Amon touches the victim's neck and forhead. Which, would be assumed to be chi blocking, but it doesn't come back. (not that we've seen)
3) Perhaps Amon isn't taking people's bending. He is of course, chi blocking at first to make them physically incapable, but I think he's really making them mentally incapable.