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.
I doubt that the administration would initiate the removal process with no outside pressure from a parent or maybe some sort of parental group a la the Satanic Panic and BADD [1].
our conversation started by discussing the definition of "ban" and people interpreting the word "ban" in a way not written in any dictionary.
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?