Both private bookstores and public libraries curate their offerings selectively. This isn't the Internet Archive, the equivalent to the Library of Congress or another institution that stores everything.
I don't know the details, but I believe the Internet Archive has some minimal filtering now as well, beyond their normal DMCA compliance. For a while they seemed to be the host of choice for extremist propaganda, ISIS videos and the like, but from what I've heard they did something to crack down on that.