Legally, it's their hardware and their OS or GTFO. that's the most obvious. Hackintosh can get you around that if you have the skills, but honestly... as much as we make fun of Ballmer for it?
Developers, Developers, Developers, Developers.
Locking yourself into one hardware and OS platform when you're in a solid third place in the desktop/notebook market does NOT endear you to the casual learner... if you don't have the beginners you'll never have experts.