As far as I understand, Amsoft only had distribution rights and when they expired the original company could do what they liked. Hence why Super Pipeline 2 came out on budget fairly quickly. Beach Head came out on Ricochet as did some other games US Gold owned the rights to until they launched their own budget label.
Yeah because a lot of the major software labels were *very* late in setting up their own labels. Kixx and Hit Squad didn't get set up until around late 1988. Prior to that, they milked their back catalogues in compilation form. I think they were waiting until 16 bit budget was in demand, which started happening around the same time.
But yeah, I don't think it was the relationship that went sour, and more a case of Amsoft knew it was time to give up the ghost and as a result, US Gold had their product back, and as you said, did what they liked.