Microsoft can do what it wants with the games it publishes. If they can make their games 360 exclusive, they might sell more consoles, which may lead to more console game sales of any game. They can then collect their portion of each console game sale. On the PC microsoft doesn't get any portion of the PC game sale unless they make/publish it. (correct me if i'm wrong on any of this).
Its a good thing they aren't the only company publishing PC games. Let them do what they want, if it works it works, if it bites them in the ass, it bites them in the ass. Microsoft is the only company in this unique situation. No other developer/publisher has to worry about competing between their own platform.