They don't necessarily have to: there's another, more comprehensive hammer they (or the US for that matter) could use: disallow vertical integration. Force hardware vendors to only sell hardware. If any software that can be changed remotely or by the user is off limits. Even firmware. Conversely, software vendors can only sell (or freely distribute) software. For vendors from abroad, don't go extra-territorial, just…
So yay, you can buy a device… and 99% of people will install the same OS on it that it would have originally come with anyway. What’s the point here?
You’d have to say desktop apps must support all competing operating systems. But, hah, good luck with that.