But it's a cost/benefit problem, no? People say this bearing in mind free speech. Sure, free speech makes it ok to say lots of terrible things, but the boon to human rights outweighs by far the evil made possible. People assume that this logic extends to technologies that facilitate free speech. Care to argue that it does not?
Because part of the purpose of these networks is to subvert government control. If you can censor one thing (e.g. child porn), then you can censor anything, which makes it useless if you're an anti-government radical in China, for example.