WHo in their right mind would get Mac hardware if they can get the software to work in cheaper hardware? Thats like suggesting that BMW should give its technology to Toyota and expect people to buy BMW's - that just won't happen. Were Apple to take that route, it would introduce people to the Mac software, but it would not do anything to the hardware side of things becasue they are never introduced to it. People would just go "Mac OS - nice, my next computer is going to be the same system - the hardware is way overpriced though - I can get it cheaper from Dell".
The car analogy doesn't really work since Apple switched to Intel. The hardware is the same. If you say that Apple can't sell a Mac because PC's are running OS X as well, then you need to explain why many OEM's can compete successfully over one OS (Windows). If people only bought cheap hardware then premium lines would not exist. Dell XPS, Alienware , Falcon Northwest, Voodoo, are all premium brands of hardware and all sell well, whilst charging the same markup as Apple and running the same OS as a $399 PC.
Not only that but Apple's OS X user base does not match Apple's hardware sales. Apple's hardware sales do outweigh OS X users, showing that people are buying Mac's to run Windows, Linux etc. Granted the difference is not much simply because Apple (thankfully) do not offer Macs with Windows installed, and many users do not know how or do not want to buy and install Windows themselves. But if Apple did offer Macs with Windows alongside Macs with OS X, I can guarantee you that Apple's hardware sales would far outweigh OS X users.
There are many advantages of buying a Mac even if you do not intend to use OS X. Excellent build quality, excellent customer support, excellent design. Certainly some Mac users would go out and buy a PC with OS X instead of buying a Mac, but this loss would easily be offset by software sales. Not only that but in many countries outside of the US, the Mac is a relatively alien platform. By licensing out OS X you would increase awareness of OS X 10x fold around the world, and I'm willing to bet that because of this, Apple's hardware sales would
increase, not decrease if OS X was set free.
If you say that Apple can't sell a Mac because PC's are running OS X as well, then you have a poor opinion of Apple hardware (which fair enough, some people do).