Ah, yes, the "I didn't know" defense. I am just blown away at how many people seem to think that because it's up on the 'Net it's public domain and fair game. Sadly that seems to be starting to be a fear among writers too. So far I haven't really heard of much fanfiction plagiarism going on, but I know that there was a big concern on the NaNoWriMo boards this year of posting original fiction and having it plagiarized.

The blanket statement really is a good idea all around. True, it isn't legally binding (at least not in Canada), but it's a start at least.