Microsoft owns Windows.But MS wants to win the console war so by putting it on PC aswell it voids people buying a X360 for the game when they can just get it on their PC's.
Except that if you look at the recommended system requirements, most people wanting the best experience for this game will likely spend more upgrading their PC than simply buying a 360. Despite the # of people with a PC, few own systems with enough hardware to play the best games at the best settings. WoW (including the expasion) has rather low requirements compared to other games out there so most current PC gamers out there haven't been needing the best hardware.
To cash in my CC rewards points for $300 in Circuit City gift cards to purchase a 360 or not: That is the question.