I know little about this. Is it the origin of a flight being from USA or the passport? I know someone who has both EU and US passports. Does that mean they could fly somewhere allowing flights from US then fly into French St Martin on their EU passport? Doesn't this defeat everything if that's possible? This same person frequently avoided the Cuban ban when the USA did not allow US into Cuba some time ago.

And MrWhiskerssammy is right, in that I've seen many incidents of American's total disregard for quarantine. If we all followed rules like the people of SXM perhaps we'd be at zero cases also and all these travel questions would be moot. (my opinion)