I don't think this was mentioned, but there recently (like this last week) was a fairly high profile case of a writer for the Huffington Post (and Columbia professor) being arrested on incest charges for having an affair with his daughter.
http://www.nydailynews.com/ny_local/...tive_for_.html
As Amazon recently had to do with bad publicity for containing books about pedophilia (or at least one of them) and ended up removing it, they probably thought maybe they ought to do that with books glorifying incest.
Once they start banning books for one reason, there are always more reasons.