If AI doesn't own the copyright, they can't make IP complaints. Gateway Inc. would have to do it.
Patents and copyright are not the same thing. Gateway own all patents, but Amiga Inc have bough the IP, which includes copyright to the OS, assuming Gateway owned that in the first place.
Also, copyright law is international law, and thus not dictated by US copyright law.
However, AI's ownership of Kickstart 3.1 rights is not in question. It seems that H&P and AI have simply been unable to reach an agreement. BOTH are guilty for allowing AmigaOS XL to be distributed without such an agreement being signed in the first place.