Re: banned books. Some are banned, some are "challenged" meaning someone tried to have them banned. The list is long and some of the reasons for banning a book may seem silly. Some of the books on the list in this latest attempt at creating a Palin scandal were banned a long time ago or in far away countries.
Re: librarians as censors. While it's true that there is a limit to shelving space in any library, the librarians who do the purchasing select according to the interests/needs/demands of the population they serve. If the book you seek is not in the collection of your library (whether it's an academic library or a very small public library), the librarian will get it for you through inter-library loan or, at the very least, let you know where it can be found. A librarian does not censor information, they make it accessible.
I realise this is the way it is supposed to work, but, where is the oversight? Most are government employees, entrenched in the system, protected by public employee's unions and the employer's fear of wrongful termination lawsuits. Call me cynical, but I have spent the last twelve years working for a government contractor and I have seen the system in action....