It's not clear whether they're legal or illegal. They're definitely installed in contravention of the End User Licence Agreement (if you don't know why then try learning about common nouns versus proper nouns), so legality rests on its enforceability. And that's a legal question that remains unanswered.

Of course, the idiots at Psystar prefer to shout about antitrust, which clearly doesn't apply in the slightest, so it's possible the issue may remain unanswered indefinitely.

Despite the fact that these are legal copies, the fact of the matter is still that they are not installing them on Apple machines, which is blatantly against Apple's EULA for its OS's.
Go give Snow Leopard's EULA a read. Here, http://images.apple.com/legal/sla/docs/macosx106.pdf.

Let's say I'm a car tire manufacturer. Now let's say that I make all customers agree to only put my tires on brand X cars (kind of like clicking "I agree" like everyone does so mindlessly). Now if a customer puts my tires on a brand Y car, I have the right to sue them, for violating the agreement.

The long and short of it is that Psystar is doing something illegal, and it's only a matter of time before they're shut down.
Unfortunately, hiding in the shadows behind Psystar are hundreds of small-time developers doing the same thing they are. The more popularity Apple gets, the more they're going to have to adapt.
The difference between Apple and Microsoft here is that Microsoft doesn't produce their own computers, Apple does. In my opinion, the only way for Apple to ever really be competition for Microsoft in terms of market share is if they allow for companies like Psystar to exist. Allowing other companies to produce machines that can run your software might not be such a bad thing for Apple. Just look at how well Microsoft is doing off of the sale of PURELY software.
The simple fact is that Apple could make a huge amount of money if they broke up their company. Apple software, Apple computers. Apple computers manufactures (duh) computers with OSX pre-installed, much like Dell machines. Apple software develops and markets its operating systems and software independent of its machines, like Microsoft.
Now, I'm no expert on the subject, but it seems like this is the only way Apple is going to make it above 15% market share before Jobs kicks the bucket.

I should mention: Don't call me an idiot if I'm wrong, because I likely have no idea what I'm talking about. ;)