The GPL was designed for non-code media. It is one of the reasons that GPLv3 has more broad language that does not refer specifically to code. Having the same license for the software and media has obvious advantage: it's a lot simpler to interact between the two of them.
Some artists just get huffy about having "their" art modified, as if their artistic integrity should not allow modifications. They would have the same problem with any free license, though, so just teaching artists how the GPL works has worked ok so far.
I'm a big fan of use-reuse whether it's code, audio, visual or whatever, but it's a bit rude in my opinion to call artists who don't want other people modifying their art "huffy."
I would like for everyone to feel comfortable putting their creative output in the commons but I respect people who aren't comfortable in that way and abide by their wishes.
I think it's undue ego. If a developer gives you code but imposes conditions that the code must not be modified because this damages their artistic integrity, (and I have seen people make this claim), most people would consider that ridiculous, because code is functional and usable. In the context of a videogame, artwork is no less functional and usable. Your ego is not more important than the people who are using your artwork for a videogame.
If you don't want to give people your artwork, then don't. But don't impose conditions that you will give it to them but feel personally offended if they modify it.
They can have all the prerogatives they want as long as they understand that their prerogatives have no place in a free game. Their artwork may be scaled, resized, and cropped in order to fit into a playing field. The colours may be changed in order to indicate teams. Additional animations may be required for certain actions. The style might need tweaks to fit the existing style of the game.
Unmodifiable artwork poses many practical problems for a free game. It's simply unacceptable.
Some artists just get huffy about having "their" art modified, as if their artistic integrity should not allow modifications. They would have the same problem with any free license, though, so just teaching artists how the GPL works has worked ok so far.