It is trivial for anyone to embed hidden iframes or send silent ajax requests to child abuse and terrorist forums without giving the visiter the slightest clue what's happening. In the case of iframes, there would probably be cached 'evidence' left on the target's pc. Try explaining that as your defence in court when you get set up by some script kiddie.
We were warned of PIDE in school for a good reason (PIDE was the Portuguese Stasi). People will abuse it.