@screw
“how can the feds have jurisdiction before the plane is even in America? The entire commerce–that’s transpiring in a different country–is under their regulation?”
The feds have jurisdiction because the carriers must certify they have met regulations and bear the cost to repatriate those denied entry to the port of embarkation at the carrier’s expense. This goes back to ocean travel. That is why visa and passports are checked before boarding and a passenger manifest is made at the departure point to be presented on arrival.
The carrier accepts these regulations if they want permission to operate into the US.