There's a lot of low-level stuff in an OS to make things work smoothly. It's impressive how Apple used to go back and optimize things for G3s, while G4s and G5s were the latest models. Each model is practically running its own OS.
So the last thing Apple needs is for their OS to run on hardware that they've never heard of. If something goes wrong because of hardware, it tends to manifest itself as a freezing or crashing machine. Psystar wouldn't be blamed for that, the users would simply think that Apple had made a crash-prone, piece-of-crap OS.