You need to think about this not on a simplistic single-game game theory model, but more specifically, creating a linux exclusive game would be part of a broade

You need to think about this not on a simplistic single-game game theory model, but more specifically, creating a linux exclusive game would be part of a broader long term strategy. Boosting the popularity of an open gaming system would reduce the rents that can be charged by network holders on gaming companies. This would, obviously be a short term money loser, but with firms like Valve, that have a major foothold and could take the short term hit, boosting a open system/network could pay dividends in reduced future costs (granted, i'm not in the industry, and this is purely speculative).