As far as I know, MSFT doesn't have a trademark on the term "window" as used to describe objects in a GUI. They have a trademark on the term "Windows" as a whole system.
Same reason you can buy "linux" laundry detergent.
I'm going to flat disagree that windowing systems and window UI elements are separate fields of endeavor, in the same way that laundry detergent and computer Operating Systems are. I think most lawyers would too.