It's not censorship.
Anyhow, trying to shut things down by definition only works in mathematics. Redefining away what Amazon as "not censorship" won't mollify anybody, as they can still choose to be legitimately annoyed at the perfectly legal and arguably perfectly ethical actions Amazon have taken. Acting legally and even ethically doesn't mean you have the right to not be criticized.
And, seeing as how if this is "not censorship" we don't actually have an English word for what it is... yeah, it's censorship. It's not government censorship, but it's censorship. It's also a sufficiently different kind of censorship that simply knee-jerk applying our attitudes towards government censorship may not be relevant, but English simply doesn't have the granularity to deal with this. That's OK. Everything works that way anyhow.
You can address that issue the same way author did it, by stating that Amazon sells stuff like "Hostel 2". Frankly, I haven't read the guys book and I haven't read Amazon's quality control policy (or whatever it's called) either, so I can't say whether the book fails the test. My point here is that most people commenting here can't say that either, for the same reasons. So why are some people so quick to side with the author and cry "censorship", when it might just be that his book is crap?
Amazon is a media outlet but they are not suppressing his speech. They are not stopping him from publishing the work or even stopping him from writing it. They are choosing not to distribute it. Thus the suppression test fails and it is not censorship.
People have been complaining about Walmart censoring media for years. I don't see how this is any different.
Amazon is withdrawing these items from circulation.
They are not withdrawing it from circulation. THEY are choosing to not circulate it themselves. The author can circulate it where ever he likes as long as it is not Amazon. If Amazon was the only place that sold books I would be with you.