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.
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.