> But somehow it’s OK for them to modify all the art assets to get the same effect of making the software as a whole effectively proprietary

Stallman believes it should be legal to redistribute all art non-commercially. That is his political belief, outside the scope of free software, however. https://stallman.org/articles/guardian-correction.html

I just don't feel like it's practical to ask game companies to distribute ALL assets semi-freely, and somehow still make a profit. Let me know if you come up with a good business model.