Yes? That's what the law currently allows. If we want to make a law that says companies are required to let end users install _any_ software they want onto any device they legally own, that encompasses almost the entire consumer product ecosystem. It is becomes hard to determine what is "general purpose" and what happens if Acer says "this machine runs windows specifically and isn't general purpose?" or they say "you no longer own this machine, you are licensing the hardware from us?"

It would not be acceptable, and it is the duty of ethical whitehat hackers to break such digital locks, flip the bird to Congress and the WIPO's DMCA, and free humanity. It would be ethical to form militias and raid federal prisons to free whitehat victims caught up by the state for it. Liberty is not free.

As long as consumers understand that is the kind of device they are purchasing then it is acceptable.

It is cheaper for hardware manufacturers to only support a single operating system instead of designing a platform to be used by many. It also makes security simpler.