Because they're not "not allowed". With very few exceptions the thing people are freaking out about is books being curated out of libraries, usually for very understandable reasons.
[1] Bothered About Dungeons & Dragons
...yes? that is the meaning of "ban", so it seems entirely appropriate to use that word
but now you are saying "the books" as if we were already talking about specific books. are you sure you are replying to the comment chain you intended to reply to?