The popular RPG isn't quite so Microsoft-exclusive.
Microsoft has their own PC-distribution system - it's called Games for Windows Marketplace - but the company seems to be recognizing that Valve’s Steam is the preferred option for many PC gamers. That's why Microsoft has announced that Fable 3, one of their flagship titles, will be released simultaneously on both the Games for Windows Marketplace and Steam.
Of course, Microsoft still wants to lure gamers to the Windows Marketplace, so they're offering a free copy of Fable: The Lost Chapters to those who pre-order there instead of Steam. A Steam pre-order will only get you the “Rebel’s Weapon and Tattoo Pack,” which features a few exclusive weapons and tattoos. If I were a betting man, however, I'd still put my over/under money on Steam as the bigger platform.
Source: The Escapist