I found fairly fascinating this post by Rachel Dukes where she follows a popular cartoon she posted and what traffic the credited and uncredited versions received. The frustrating thing about this is I'm not sure there's much you can do except note it -- but it's also extremely important you make the kind of protest she's making. One of the big canards of digital culture is that certain behaviors are locked in stone and unchanging; this is so untrue it's not even funny, and you start to notice such changes if you pay attention for more than a few years at a time. I've been putting material up on-line since 1994 and the way and the extent to which certain material has been used over the years has changed a lot. So keep pushing; change isn't just possible, I'd say that change of some sort is inevitable, even when it comes to the way people use other folks' material.