Of course there are some books not appropriate for young children. And parents should be aware of what their children are reading -- should maybe even read it first rather than make an uninformed decision. But things get dicey when one set of parents wants to decide for everyone.
As a lifelong reader, the thought of not being allowed to read something, just because someone else decided I shouldn't, is anathema. And I salute the brave librarians who stand up to those who would attempt to limit others' right to read.