Simple for money. No doubt it will require an online confirmation to make sure your Windows is legit firstly.
But people playing future Halo's and Gears of War's on a PC is a big selling point for Windows 8 if they do indeed go ahead with 360 compatibility. And like I said above developers have the information to inject that into future games.
Its an obvious money maker, why be content on getting someone to buy a 360 when you get them to buy Windows 8 at the same time :P
It will also increase sales for games also, and as said since Microsoft acts as a publisher for developers who need one, they can get some profit from that also.
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem