I've owned and used microwaves since 1977, and indeed the majority of them have worked this way. Not all, however. But I don't think the variations from this

I've owned and used microwaves since 1977, and indeed the majority of them have worked this way. Not all, however. But I don't think the variations from this de facto standard have anything to do with patents. I think it's more the "NIH factor" ("Not Invented Here").