The likelihood of any legal restriction was probably close to zero - it’s only from today’s era of hyper-regulation that we might even imagine something like that. Most likely it was a deliberate technical limitation. After all, dialog windows themselves were already overlapped. I remember well what a headache it was to program and render graphical elements on those old machines (PC AT 80286 with 256 KB of RAM).
How is regulation involved? The OP's idea was that Microsoft feared being sued by other companies over IP. If there is some increase in lawsuits between companies, I'd like to see evidence.
Also, my understanding is that there was a lot more regulation in the 1970s and extending into the 1980s, though it was probably declining by then.