This is because most libraries encourage controversial books. Historically, books that were banned, such as Moby Dick, Huckleberry Finn, are well written books from an English standpoint. Television is much more "racy" and "should be banned" than books. Though books pull you into the moment a bit more, they should not be banned, or else we are like Russia.
Libraries are not solely responsible for banning controversial books. The responsibility should also fall on people who use the public library and other educational leaders who notice the controversial books. Even then, one must take into careful consideration whether or not the books are damaging to the society or the people who read it and to what extent.
It is not the business of libraries to regulate what their patrons should read. Libraries should be an open and encouraging place for many ideas including controversial ones. The role of libraries is to provide information on many topics not tell their patrons what knowledge they should pursue. This is important as to not infringe on the peoples rights.
Libraries are a bastion of knowledge. The public should have access to literary works that not only reinforce mainstream ideas, but offer challenges to those same ideas. If someone reads about a challenge to a mainstream idea and finds flaws in that challenge, then their preconception is justified. If a person finds a challenge to a mainstream idea and feels it requires further thought or discussion, then the expansion of the conversation helps everyone involved understand. That is what controversial material is for. To keep the reasoning behind certain things be more than "because that's how its done."
I personally think that libraries are not responsible for banning controversial books because it is their job to stock up on books and making sure that the libraries is well kept up. I personally think that libraries are not responsible for banning controversial books because it has nothing to do with librarians.