It's probably a breach of contract-- and since the seller is acting outside the scope of the license (End User License Agreement) it may also implicate various intellectual property infringement issues, including copyright and trademark.
Apple would say it is definitely illegal, in the same way they say that jailbreaking the iPhone is illegal-- Apple likes knowing how their devices and software will be used by purchasers and they make that clear in their licensing models.
There's dispute as to whether such provisions are technically legal, and it's generally accepted that in most cases you can "sign away" certain rights (such as rights associated with how and where you install legally purchased software) provided the EULA is appropriately clear and understandable by licensees. There is no bright line test with regard to the legality of such provisions and it's typically determined on a case by case basis.
Hope this helps!
There's a whole lotta issues in your question, so it's impossible to give a definitive answer. Some EULA terms are simply unenforceable (for one reason or another), so nothing will happen to someone who violates those terms. Some EULA terms are enforceable, but only on a contract basis, so if you violate them, you'll probably only have to pay money damages, and/or lose your right to use the software. And some EULA terms may connect to copyright, so violation of those terms would constitute copyright infringement. Of these, only copyright infringement is actually illegal. It's even possible, but not common, to go to jail for certain types of copyright infringement.
As to whether it's against the law to sell a system "with" Mac OS, well, that's the Psystar suit. That's been going on for a while, and I haven't really been following it, but it looks like there's a hearing on 14-Dec-2009 where some important questions will be argued. When those arguments are decided (probably shortly thereafter), we may find out how one important court views this question.
If a person buys software and loads it (or permits it to be loaded) onto a computer the person has agreed -- via the software license -- that it would not be load onto, then the person has breached his or her contract with the software manufacturer. I don't think that wrongful conduct, however, also infringes any of the copyright rights that protect the software (in light of 17 USC 117). [I'd be happy to be corrected on that point by someone more knowledgeable on the subject than I.]
So ... would selling the computer be unlawful? The law requires parties in contract to perform their obligations under the contract -- and anyone who does not is liable to the other party if that party is damaged. So, yes, it's unlawful to use the software in any way that's outside the scope of the license and, therefore, selling a computer that's running unlicensed software would be unlawful (as a breach of contract, though not an infringement of copyright).
The harder question is whether it's unlawful to BUY the computer. Unless there is some copyright wrinkle of which I'm unaware, the only claim I can think of that may apply is interference with contract. So ... maybe it's unlawful to buy the computer as well. Tough call [at least for me].
Computers are very reasonably priced nowadays. It makes little sense to wrangle around with ones that aren't running properly licensed software.