Like Painkiller said, instead of worrying about copywrights and the like, we should instead just focus on the ethics of it.
No need to bring lawyers into this.
edit: There was no "stealing" involved in the compilation mod as the author never claimed it was his, he gave credit. He just didn't ask for permission.
We also gotta stop acting like "mine mine mine". Instead of thinking individually we should all work to make MW better.
I agree, it's all about what happens to it in the end, if someone's taken your texture and manipulated/used it to make a wonderful peice of art, but not credited me, I wouldn't care. I'd probably still ask the author/user about it, but I wouldn't want it removed or anything.
An example to this is a youtube user by the username of sarakling76 http://www.youtube.com/watch?v=XU2EtLHVoiI, then another user by the username of DigitalPiece http://www.youtube.com/watch?v=8coPXbrIvcg made from the clips of that video. DigitalPiece had contacted sarakling76 about it, without response, so uploaded the video anyways.
If I were sarakling76, and I found out about DigitalPiece's song, I would let him keep the video up because it is a unique piece of art and deserves to stay up because obviously some work was put into it and the video can't really be claimed (ethically and artistically, in my opinion) as hers.
For other situations though, like if someone took a texture and put it into a game and sold it, because they thought it was "free", I would be pretty upset.