Libraries buy books based on community interest. That’s why they usually have many copies of the latest best-seller, and very few copies of books about, say, getting a ham radio license. When people in a community are interested in Marxist theory–and face it; very, very few people have read Das Kapital in it’s entirety, fewer have fully understood it, and all of them have degrees in philosophy–the libraries will buy books on Marxist theory. When people want to read Walter Benjamin or Mikhail Bakunin, the libraries will buy that. When people want to read Ayn Rand (who is even less entertaining than Marx, and she’s trying to be entertaining), libraries will buy that.
My library has exactly as many copies of Ulysses as they do Das Kapital, and I’m in a very small town in the deep south. No one cares of banning James Joyce. Space on library shelves and funding is limited, and libraries don’t usually see a point in filling up that precious space with, and spending money on books people ‘should’ read that are never going to get checked out.