Obviously they pick some books and not others for some reasons. If you like their reasons you call it curation, if you don’t like their reasons you call it banning.
Randomly selecting books for rotation would bias by sheer publication volume. I don’t know about you, but I don’t want my local library to be 40% Atkins Diet by volume, regardless of how positive I might feel about it.
Nobody is talking about pornography; not everything that contains sex or violence is pornographic in nature (much less obscene).