EULAs generally fall under contract law / copyright law. "By following these rules, we grant you a license to use a copy of this product" is the name of the game with a EULA. Contract law and copyright law do give copyright owners and those who entered into a contract a means to find relief if one party violates copyright law, or violates the rules of the contract. Also, remember that the GPL is a EULA, using contract/copyright law to the advantage of the OSS community. It is also fairly open in nature, and as such, less likely to run afoul of the problems below. But for the GPL portion of the OSS community to thrive, EULAs need to be enforceable on some level.
Usually were EULAs break down is if they have rules that cannot be followed without breaking local law (which can plague any contract), or those rules are specifically prohibited by local law. Another issue is if the purchaser buys the product, sees the EULA and decides not to be bound by it, and then cannot seek a refund from the seller and the seller auto-assumes the buyer entered the contract. Coercion into contracts is bad, and can get the contract rendered null and void depending on your local law. This is a fairly hotly contested area of law right now (what renders a EULA null and void and what doesn't, and why, and if bad clauses invalidate the entire contract, or just the clause, and what jurisdictions does that happen in).
The judge saying Psystar can make a case doesn't mean that he has an opinion at that point, but rather that Psystar may have a legal leg to stand on. If there is valid reason to hear the argument, it must be heard. If someone comes in and their claim is basically gibberish, the judge is within his/her right to dismiss the case right then and there. But as law can get murky, a lot of cases do get heard regardless of the judge's personal opinion (which shouldn't even enter the equation, if the judge is good at what they do). SCO's own lawsuit against IBM was ridiculous, but it had to be heard, because there may have been a legal leg to stand on, and it wouldn't have become even remotely clear until after the discovery phase.
I personally would have bet money that Apple's winning the case would have hinged on the fact that Psystar was pre-installing OS X on machines. Doing so would have meant Psystar was entering the contract on the buyer's behalf. If Apple's EULA clause that it needs to be installed on hardware that they brand is valid, then Psystar violated the contract on behalf of the buyer and could very likely be held liable for that violation.