Well one point for Apple not supporting install of OS X on all computers is one of driver issues - everyone makes windows drivers so windows does run everywhere, and linux can afford to be a bit more buggy at drivers (or harder to setup) since it's usually free and community-maintained, but not everyone makes mac drivers and Apple can't write them all by themselves. And they couldn't advertise mac OS X installs on the average PC without drivers, cause users would try it, it wouldn't work, OS X would get a bad reputation, etc. that's how they achieve the rock stability that makes OS X users love it
HOWEVER, I think *forbidding* it is totally abusive. They could just have marked it unsupported (and of course it's more money for them this way)