I've reported on the following before, and got generally booed down by the Anglosaxon/Texan crowd, but I'll try again. Perhaps with a different lead-up this time.
But before I do, just one remark/opinion: The aspect of international passengers is probably less a legal than a diplomatic problem. Foreigners travelling abroad are subject to the laws of the host nation; so it is legal under international to, for example, sentence and punish a foreigner to death if he has committed a crime in the host country that is punishable by death in that country. Only thing is that, by way of treaties, those foreign nationals enjoy a few privileges, such as access to assistance by their home country consulates. It's just that executions of Europeans frequently raise outcries over here - but that is not a legal problem.
The action of the hijackers is best classified as a crime, not an act of war, and thus subject to domestic criminal law and the domain of police powers. It is, as such, not subject to international law, or the rules of war.
Ok, now on to the legal arena: As far a I am aware, the question of whether or not the goverment can order or authorize the shoot-down of a civilian airliner has not been tested in any court of law - except the German Federal Constitutional Court. True, German law doesn't apply in the USA; and continental law traditions differ from Anglosaxon ones, but I don' think the differences are so great that this precedent isn't worth cosidering here.
Following 9/11, the German government realized that it could not have responded with military force to such an emergency, and felt that another attack like 9/11 was a distinct possibility, and thus felt the urge to give itself the legal power to order shoot-downs (if I remember correctly, the idea was to arm police helicopters with air-to-air missiles, as use of the armed forces domestically is quite a taboo). So a law was quickly draftes and passed that would give the government, under specified condition and by specified individual offices, such powers. The law was promptly sent to the Constitutional Court, and in short order struck down.
The Court reaoned (this is from memory, so probably neither complete nor entirely accurate - just to give you the drift):
- Citizens must never be mere subjects of government action, without regard of their individual personality and rights (that's the Court's interpretation of the "dignity of man", the protection of which is the state's supreme obligation, according to Article 1 of our Constitution)
- Ordering a shoot-down to prevent a crime and potentially save others would ensure the killing of individuals, while expressedly not considering them individually. Therefore, the order would violate Article 1
- The Court considered that the government can't predict the future, and can never be 100% sure that a hickacked plane would really be crashed; there is always a chance, however minute, that the plane is retaken in time, or the hijackers change their minds, or they never actually intended to crahs - and the passengers survive. So the government would take a decision to take away their chance to survive, thus violating Article 2 of the constitution, which posits that all powers of the starte must protect the lives and well-being of humans (this Article in theory allows for exceptions, but none that apply here).
- As for the lives of the passengers on the ground that the shoot-down would aim to protect, the Court opined that the state cannot weigh life against life, not even unequal numbers of lives against each other, again citing Article 1, because a decision based on mere numbers would again disregard the individual dignity of the potential victims
- It is recognized that people die at the hands of criminals, and that the state cannot fully protect all people from such harm. This is a reality that the state (and her citizens!) has to suffer, and always has - it cannot go to any length it wants to remediate that situation
- Self-defense is a right that can't be applied here, as it can only be directed at an offender, not at by-standers or other victims.
While the US Constitution doesn't have the German constitutional Articles 1 and 2, I am sure that some of the reasing would apply there, too. Surely, it is well embedded in the American legal tradition that individual rights must always be considered, and that the state's power to tread on them must be reined in tightly. And equally surely every American would recognize that it is not a good idea in principle to give the government the explicit right to shoot innocent citizens. Just imagine such a law had been proposed and enacted pre-2001. What would CTers have made of it??