So your opinion is that a bookstore should be morally obligated to carry every single book that has ever been presented to them to consider for sale, and that if they decline to carry a book or genre or some other set of books they're engaging in evil censorship?
Please, don't use words as "evil" as some sort of binary moral value. I'm sure that most people at amazon are quite torned regarding this. It's even understandable in their positions, despite being completely aware of the possible consequences.
Unfortunately amazon has such relevance in today culture that the effect of their censorship is just nominally different from a violation of first amendment from the US government.
I'm not from the US, but from what I've studied the constitution was created protect the population from what was considered the only possible danger for the population as a whole (i.e. the English Monarchy or the United States themselves), from a time where private companies weren't considered capable of doing the same.
You'll forgive me for scoffing; I find it absurd to pretend that Amazon's main customer demographic cares for even a split second that Amazon stopped selling gay rape fantasy books. I would venture to say that most of Amazon's customers, if they even hear about it (which is doubtful), would simply shrug and say "Good, now I won't accidentally stumble on one."
Point is, Male on male sexual violence is depicted inside the Divine Comedy,the Decameron, hinted in the Tempest, and most of the Greek neoclassic tragedies. Actually, gay rape is somewhat tame considering what happens in most tragedies. It's not unknown in french literature, too.
If you're looking for something more "Pop", there's always Fight Club (the book, not the movie), and most of the "Punk" literature movements starting from the '90 in UK and Detroit. Some of these books have been awarded by the New York Times as "book of the year".
Yeah, I'm perfectly conscious that the books here considered are not even close to the cited works. And as I said before, I'm not a psychologist, so I cannot even fantom the real effects of such books. Problem is that such books are still sold, and the writer of the censored books would be entirely in their rights to ask "where is the difference? why aren't they being censored, too?" ; speaking in terms of contents and not artistic merits, of course.
But by using such a broad brush that is censorship, Amazon is basically answering them , "because I say so", which summed with the point I've made before, creates an ugly, ugly precedent.
There's the plain old strict definition of censorship and there's the evil freedom-of-speech-suppressing information-hiding most-commonly-used definition of censorship. We really only need to worry about the latter.
Unfortunately they only diverges in their means and not their mechanics. Censorship it's still censorship, and should be applicable only in cases where it's objectively clear that rampant distribution would be a dangerous thing.
Side note: I live in Italy, a country well known in present day for the effects of lack of separation between the private media and the political power. Previously, it was known for fascism. My grandfather used to tell me that fascism didn't have to get control of the press through violence for a simple reason: the whole production of paper for books and newspapers was owned by Mussolini sympathyzers.