The thing is that there was no USFL. It was officially abandoned in the eyes of the law and FOX knew that. I haven't seen any recent news on the lawsuit, but definitely interested in what the outcome is. I think if anyone looks dumb here, it's the original USFL owners for giving up all rights to their league and now trying to cash in as soon as someone else picks it up.
On FOX's end, it was smarter to draw from a well that was already dug, rather than dig a new well. It's hard enough to get these new leagues started, so you need to cut corners wherever you can.