If Apple owns the OS, why should they not be able to restrict it’s use?
They may own the rights to the OS, but that shouldn't matter in this case. From what I know, PsyStar was simply buying legal, retail copies of the MacOS and installing them on their products. I see nothing wrong with this.
The doctrine of first sale should, in a good world, trump any of Apple's claims.
Doctrines of Fair Use, First Sale, and the general principle that copyright is not absolute.