Waaaay back when Windows 3.1 was out, they didn’t care HOW you got it. They even provided free 1-800 phone help, based in the USA, staffed by Americans and they didn’t ask how you got it.
That’s how they built a benevolent monopoly.
...and before that Windows 1.n was given away free with the purchase of 4k of memory chips iirc.
Windows 3.1 ran on top of DOS. The first version of Windows that was arguably an OS was 95.
True, but that monopoly is not so stable now. It's still a huge majority, but it's viewed with more suspicion and dislike, and even Microsoft recognizes that their customers see that they have alternatives that are viable. It makes a big difference in how they act.