The following is a common position in the libre software community: "Proprietary software is not acceptable, but proprietary non-functional data is fine." This is the position of the Free Software Foundation. Personally, while I think proprietary artistic works are unethical, I basically adhere to this position in practice, because I don't think boycotting an artistic work does any good. To have a meaningful effect on a culture, you must be a part of it, and unfortunately, the vast majority of our culture is copyrighted and proprietary.
However, it seems to be a common mistake for people to take this permissiveness excessively far for games, to the point where lines are drawn in arbitrary places concerning what software is. It can be very easy to say that a game is "entirely libre software, but has proprietary data", and therefore conclude that the game is fine. This is an oversimplification. In fact, it is quite common for "data" in games to include software, such as scripts, and so a lot of games that seem on the surface to be libre software, in fact, are partially powered by proprietary software.
To address such mistakes, this article lists cases of "gaming traps" – games which contain proprietary software that you're likely not to notice, usually because of software that is a part of the game's data. This is permanently a work in progress, as it cannot realistically ever be truly complete. If you know of another example, please let me know so I can add it.