The article gives several examples in the U.K. of political attempts to ban content and the difficulties, both legal and technical, in actually establishing controls.
"Blocking websites also brings into play the incredibly labyrinthine arguments around just what is terror-inducing material. For example, a website that trumpets itself as a holy warriors' resource, complete with instructions on bomb-making, might easily be called a terrorist site and treated as such under law. However, what actions could—or should—an ISP take on an arborists' informational site that includes instructions on how to make a stump-blowing charge of black powder? Or is that arborists' site a front for a terror organization?"
Terror on the Internet: A Complex Issue, and Getting Harder