I always see PC/360 exclusive as a "Microsoft" exclusive considering most PC's run on Windows. Wikipedia claims less then 1% of computer users have Linux and Apple is at about 5%. People can argue that Gears of War is not a 360 exclusive (well the second one is till they put out the PC version) but It's still a Microsoft exclusive and your gonna have to buy their OS most likely to get it.
It's just that simple.