"Odd that Apple won't let Mac OS be sold as an OS for a standard PC."
Yes, I have yet to see a convincing explanation other than than one or both of the following:
1. Apple do very good business from their hardware sales and wish to discourage customers just buying the os and doing their own thing with regard to the hardware.
2. They do not want a situation where straight sales of OSX per se could be compared with straight sales of Windows. (By straight sales I mean full version machine independent. Not pre-loaded, that would give Windows an unfair advantage in the comparison).
I am inclined to place more weight on the first of the two.