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).
Apple sued Digital Research and later Microsoft when they enabled overlapping windows for windows 2.0.
Also lol a 286 with 256kb of ram - that is a very very weird combination you would never see in a desktop. Generally early IBM PC compatibles might have just 512KB of ram but around 1985 and later 640KB really became the norm even on 8088 and 8086 based systems. I am not counting stuff that really didn't get anywhere like the PCjr and that thing was much earlier in 1983.
286 based systems once they became more common started a 1mb RAM.