Canonical (for obvious reasons) continues to celebrate proprietary games made available to Ubuntu users through their Software Centre. I’m not entirely against proprietary games for GNU/Linux, though I won’t install/buy this title given its licensing structure. OilRush is based on the proprietary Unigine.
I think a better balance with free software would be to license the game’s engine freely and distribute/sell the data sets under non-free licenses. Though that would irk Free Culture supporters, they’re irked either way. A free software engine can be an incredibly useful contribution to computer science and other software that puts the engine to use.
Exceedingly restrictive licensing for GNU/Linux games however, appears to be an inevitable trend in the short-run. An optimistic view sees software companies planning their long-run under the assumption that freer software licensing will offer a competitive edge and maintain profitable opportunities. If this were to happen, it may prove the OilRush model isn’t sustainable.