Amazon can't ban books. They can, however, not sell them. Yes, in America there is such a thing as freedom of speech. Just because someone wants to say something doesn't mean that any corporation should give them a stage to say it from.
The people calling for the removal of this book aren't saying they want a stop to any sort of sexual expression between two or more consenting adults. They are saying stop trying to make a buch by giving the perverts an execuse to screw our sons/nephews in the form of this book. If Amazon stops selling it people can still purchase it elsewhere if they like but people who believe this book is wrong will not support any corporation who does sell it for moral reasons.
What's so horrible about withdrawing financial support from a corporation that sells something you disagree with? Yes, it has been used by fundies numerous times but just as the people you disagree with have the "right" to say what they want, you have the exact same right to say you don't like it. Withdrawing your support from a corporation you disagree with because they are selling a product that supports child molestors is a person's right.
Sure, people have the right to go on and on about how people of a different race are somehow less human than mine, they can talk about how horrible gay people are, they can talk about how they wish Hitler would have killed off all those damn dirty jews. That doesn't mean that I have to let them into my house so they can spread their message any further. Same with Amazon. Pedophiles have their right to try to explain their sickness but that doesn't mean that Amazon has to distribute their literature for them and I know for one I wouldn't step foot inside a store that does.