common sense
I suppose that no commander of any armed forces (of a civilized country, that is) will dare to order a shootdown of a commercial airliner whose crew has trufully and repeatedly said the 'e-word' and stated the nature of its problems and intentions. Even after 9-11 I find that hard to believe. Given this assumption, when a pilot of an airplane in distress does not allow a 'less cooperative' air traffic controller to intimidate him/her, it's fair game according to 14 CFR § 91.3 ('In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.') to insist on landing at a US airport (maybe not so close to downtown Manhattan) and start descending.
If I am not mistaken, any passenger who stays onboard an airplane will - according to internationally accepted diplomatic rules - technically remain the territory of the airplane's state of registry, even if the airplane itself stands on another country's soil. So, US authorities would have no jurisdiction over them.