I'm just thinking, that Apple makes an awesome OS and only a small amount of people get to use it because not everyone can afford a Mac, so they have no choice but to buy a low end PC. If Apple made a PC compatible version of OS X, and sold it at a higher price (because OS X is cheap since you pay for hardware), maybe OS X would spread to every PC and everyone would be using OS X instead of Windows... And then Apple would be rich enough to make cheaper Macs, so eventually everyone would buy Mac hardware as well... In this sense, OS X would be directly competing against Windows... Because right now, it's not, since it only runs on certain hardware, and Windows runs on all hardware.
Then of course I know, this would involve Apple having to deal with all sorts of compatibility issues, and OS X might end up looking like Windows... So maybe that's why they don't want OS X to run on anything but a Mac.