Restricting Books? Really?

Nick shared this article on Facebook about parents wanting the Hunger Games books pulled from library shelves.  While I haven't read the books, I just cannot get past the idea that we still want to censor books.  Kids have easy access to violence and porn on the internet, so why would we stop them from READING something even if it is as graphic as what they could see online.  Is it that these parents do not value literacy?  Are they afraid of exposing their children to ideas outside the parental comfort zone?  Are libraries required to remove books if a certain number of removal requests are made?