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…
What if we allow hardware manufacturers to create software to run their hardware but open the schematics and make the hardware hackable such that others can run software they want? That seems like a decent compromise because there’s very little margin in hardware alone and if you force hardware makers to go that route pretty soon there won’t be any because they’ll all have gone out of business.
Allowing hardware manufacturers to ship their own software, even if that software must be free, even if the hardware specs must be released, will still allow them to ship crappy hardware interfaces and compensate that with bloated software. I’m also afraid some manufacturer would try to game the system, with, say, tweaking the instruction set of every new GPU so third party drivers can’t really keep up.