The issue of whether a vendor can impose additional restrictions via EULA is a hot one. Apple lawyers say the license terms change the consumer's legal rights once they accept them. On the other side of the argument that puts Apple in an unfair position to require things that are unreasonable because otherwise consumers can't use the software they purchaed.
However there's a lot of debate in the legal community about it, and cases must really be decided on an individual basis. While there's a history of EULA's being upheld in court this case revolves around an unusual restriction. It's far from clear whether Apple will prevail. In fact htere are a number of good arguments against them.
Apple's attempt to control what consumers do with their software after purchase is questionable at best. Just as copyright holders have certain exclusive rights, so do property owners. Once you've purchased a piece of software the license has limited legal force beyond very basic questions like how many computers it can be installed on simultaneously.
Besides the copyright aspect of the case there's the question of whether Apple's license terms are monopolistic. The appearance that the EULA seems designed to limit competition could be a key factor in whether they are shot down by a judge. A restriction like on something like the number of computers you can install a piece of software on is a key factor in the company making enough money to cover development costs. This is a standard part of doing business. Restricting what the buyer can do with a product isn't.
The other claims, for trakemark and trade dress infringement appear intended to give another justification for the anti-competitive license terms of OS X. The argument goes something like this. By installing OS X on their computers, Psystar is giving consumers the mistaken impression that the clones are actually made by Apple. Apple's lawyers argue that since that's the only computer consumers are used to OS X being available only on they believe Psystar is selling actual Apple computers.
In other words they're claiming a previous lack of competition gives them a right to disallow future competition because consumers aren't used to it. There may be some compelling arguments for allowing Apple's restricted EULA provisions, but if this one is close enough to the top of the list to merit mentioning it in their legal argument I wouldn't be so sure.