The difference is that everyone agrees with file sharing, only the RIAA etc etc disagree with it. Child porn is something that mostly everyone disagrees with.
The majority wants to legalise file sharing, and the majority wants to get rid of child porn. The majority decides. ISPs should NOT be forced into blocking certain services on the internet by a small group of people with a lot of money. The web contains an endless amount of information, which should be free for everyone to read. They have no more right to force blocking than I do, just because they have more money than me.