I don't think it's the fanart itself people are worried about. It's the fact that the artists themselves are so anal that they shut their entire site down if they see it happening to their own stuff even once. It doesn't happen all the time, but it seems pretty common with Japanese fanart sites...

Basically, this whole shit is because one person decides to take art of someone else's character some bitchy fan drew and therefore no one else gets to see it. Round and round the circle goes, where it stops, nobody knows.