here's where the separation kicks in. "the game" is a combination of "the software" and "the art". the software is free, even if it's not usable on its own. it's desirable that the artwork be free, but there's not a moral imperative that it be so. alternative artwork can be developed and used and, till then, the game software is like any other partially-implemented free program, still missing essential features, but freedom-respecting nonetheless.

I don't know where the notion that OSS permits proprietary artwork comes from, but it strikes me as incorrect. in the FS movement, we have a philosophical and ethical basis to our stance, that leads to software and documentation that respect the four essential freedoms for works that do a practical job, and to artwork that is at the very least redistributable. OSS, despite having started from an equivalent definition, doesn't share those ethical values or principles, and though its practitioners are tolerant of software that doesn't respect their freedoms, I see nearly no evidence that those are considered OSS (pretending Linux is OSS despite the blobs being the one exception). now take, for example, Debian's stance, based on their own FS definition, that later became the OSS definition. it leads Debian to qualify docs and whatnot as nonfree, but to accept, maintain and supply them anyway, but *not* to consider them free. AFAICT OSS does the same.