Essentially when microwaves were introduced the 2.4GHz spectrum wasn't allocated from my understanding, and USA proposed the current allocation as global one so that nobody would try to depend on it and then suddenly face interference from airliner's microwave oven (for example while on the ground) - this move preempted various legal issues of including the devices in airplane equipment and probably made for easier spread of microwave ovens in general as they got cheaper (because less certification etc.)
Thank you, and I hadn't known that part of the history. I knew that microwave ovens could cause issues, especially for older models with poor shielding or if the door is opened prematurely, but hadn't realized that was the reason it stayed unlicensed.
