Porting from consoles to PC's is much harder, though, not to mention that they'll also have to upgrade the graphics in a major way to make it a "competitive" PC game. By contrast, porting to Linux should be much easier.

Also porting old games is one thing, but these days most 3rd party game engines support Linux, too, and it should be even easier to support Linux at launch with new games.

Considering Linux users buy about the same amount of games (in revenue) as Mac users right now (from the surveys/research I've seen), it should be a no-brainer to support Linux if you're already going to support Mac. I could see some not wanting to bother with Mac either, though, but for those who choose Mac, they should also choose Linux.