There is the small matter of a 2-week quarantine upon return though.

Just because citizens can legally return does not mean that doing so is viable or economical - the government has no legal obligation to make sure there are 1) flights to and from the US that 2) are affordable. Those are going to be the big issues.

> It's kind of unconstitutional to stop them from returning.

Sort of. The trick is to get the airline overseas to refuse boarding. (Probably) not unconstitutional because it's not US soil.

Just like how the US directs airlines overseas to refuse boarding if you show a US birth certificate and photo ID instead of a US passport.