I'd also add that a Creative Commons license is NOT the default at Flickr; you have to explicitly change the license to that.  It is possible to change one's user setup there so that everything you upload gets marked as the Creative Commons license one chooses, but again, not the default, and too much effort in general for someone uploading copyright violations.

A bigger problem would be people uploading stuff that's made freely available by someone else but under a Commons-incompatible license, but has been incorrectly marked as the 'nearest matching' CC license, out of a desire to display its free status.

-Matt