Indeed, why not promote/create the game for their own products like WP. It seems really strange to promote a competitors product in your own products.
Is Microsoft really competing with Apple when it comes to gaming? In the end, it's all business. You weigh the pros and cons. Sure you'll sell more windows 8 products, but imagine all the potential sales if it comes to mac and iOS? There's no reason it can't come to all platforms.
Same reason Google allows it's apps to be on the iOS ecosystem. Same reason BBM from BlackBerry is finally coming to Android/iOS.
Microsoft had no problem releasing a few games on Game Boy Advance and DS that used their characters or IP.
It'll just be weird seeing Halo 'everywhere' in a sense. TV show, novels, comics, all kinds of games on various platforms.