Librarians claim to hate the idea of banned books. The ALA even promotes a yearly "Banned Books Week" that celebrates our freedom to read whatever we like. Libraries set out all the books in their collection that have ever been banned, proudly displaying their open mindedness to the community, inviting us all to be rebellious free thinkers.
But there is a subtle form of censorship in libraries. In the same way that the winners always write history, it is the librarians that make up the collection. They are the ones who ultimately decide what knowledge is readily available.
For example, if you go back in the stacks at the library where I work and browse the 900s - 973.931, to be exact - you'd think that conservative writers didn't produce much of anything during the last eight years.