[jcranmer]

> We can't prevent US citizens from returning to their homes.

SCOTUS has ruled that the US does have quarantine powers for medical emergencies, even for its own citizens. Maybe a complete ban if poorly orchestrated might run afoul of the Constitution, but a policy like "all travelers [US citizen or not] from X region must present at <specific port of entry>, whereupon they will be transferred to a quarantine facility for 14 days" would totally be fine. Note, for example, the way that the quarantine on dogs because of rabies is being handled.

> But we can prevent others from traveling to the US. It might make sense to ban all passports except the US for flights originating from China, but then you end up dragging in connecting flights through China from other countries.

Why should you exempt people whose only presence was via connecting flights? This generally involves long layovers inside of airports, where a large enough fraction is potentially susceptible to already be concerned about (due to local people making their flights), and you're likely to be spending a decent period of time on the plane with such people as well, too.