Flickr and Creative Commons

I’ve never given it much thought as I’ve tried to be consistent, but Flickr allows users to switch back and forth between Creative Commons licenses and traditional copyright, despite this being in violation of the CC license itself. Most of my work has been published under a Attribution- Noncommerical, Share-alike CC license since I first learned about copyleft several years ago. And though I’ve never really considered degrading the license back to full copyright, apparently other people have done so. I don’t know of any legal fights over this (yet), but it seems like it would be a nearly impossible position to defend.