Redux
1) Apple has a copyright on OS X and its software. In other words, they have a right on every copy and how it is used. This would be true (in the legal sense) since Queen Anne's day ... assuming software was made back then.
2) As such, Apple can put their own terms and conditions on the sale of the software. They can give it away for free (a la BSD, Linux). They can sell it with a service package (Red Hat, etc). They can sell it to any and all who sign a contract (MicroSoft) or any select group. Or they can be the only ones to sell it, which is what they do, on conditions and terms they set, which they do.
3) OS X is available for purchase seperately from their hardware.
4) If you have a spare box and want to purchase OS X and try to install it there so you can try it out, Apple won't come after you. Remember, Jobs and Woz were hobby-ists, that's how the company was founded. And if you are buying OS X to try it out on old hardware, that's another copy they sold that they wouldn't have otherwise, and you probably wouldn't have bought an Apple machine anyway.
5) Just don't expect any support from Apple. If I tried to put fuel injectors from a Ford Focus into my VW Golf, and things go badly, tough. I can't sue VW nor Ford, nor can I expect much sympathy, because I was using the two products in ways they weren't intended.
6) If I then try to re-sell that machine with OS X, then I'm in violation of the terms of agreement of the sale of OS X to me. Remember, Apple retains the right to that copy (just as MS would with a Windows install and Torvalds would with Linux). Apple has chosen not to allow such resale, and for a good reason. The clone program of the 90s were nearly the coup de grace for the company.
7) Psystar violated the point in 6). And they continued to do so after a desist order.