Amazon is stupid, but it turns out they didn't do it on purpose.
There's a little-known "flag as inappropriate" link on all Amazon content. If it gets enough votes, it gets pulled from searches automatically. Amazon was being euphemistic and calling this "adult" instead of "inappopriate" in their auto-replies.

This has been a disaster waiting to happen for years (hence Amazon is still stupid, they could have prevented this from ever happening a long time ago), and somebody finally set up a script to mass-vote entire categories. That's why "homosexual" had stuff banned, but "erotica" didn't, because he didn't select that criteria.

This guy has taken credit for it. And even if he didn't do it, the method described is entirely plausible.

Again, they're still stupid, but they're not bigots. And yes, the fact that this has been going on for years with no opt-out to view the filtered content is quite annoying.