It's implied in any claim that American citizens cannot be killed by US armed forces overseas.
For this "right" to be implemented, fairly obviously a hearing must be held to determine citizenship status before a soldier returns fire.
If US citizens aren't to be targeted, how do you know whether a possible target is a citizen without such a hearing?
BTW, the above is an example of trying to carry an argument to its logical conclusion to demonstrate its flaws. However, I see no reason why those planning and launching attacks on us should be treated differently based on citizenship status. If we catch em, we can try them for treason in addition to any other crimes, but that's about the sole distinction I can see.
I agree with you about extra-constitutional powers being dangerous.
In 1901 the Supremes ruled that the Constitution does not always necessarily follow the flag, in Downes v. Bidwell, even when the government is in control of territory. How much less does our Constitution apply in foreign countries?
Nobody claimed that US Armed forces cannot kill American citizens — you have obviously never been in the military.
If someone's shooting at you, what do you do? Shoot back!
You always have the right to defend yourself, your fellow soldiers, and allied forces.
Heck, even a jury-trial would find a soldier innocent for returning fire and killing the attacker.
But all of those are fundamentally different from the issues being discussed here: if the government can paint you as 'terrorist' and use that to deny such things as the 4th, 5th and 6th amendment requirements, then how long will it be before protesting the government will itself be punished by death? And they can do it in subtle ways, with the NSA's domestic-spying, the IRS's capricious rule-changes, the DEA/BATFE/FBI/ICE's willingness to conduct the state-sponsored terrorism (and likely easiest Treason case in 100 years) known as Fast & Furious.
The War on Terror will finish off the last vestiges of liberty and rule of law that the War on Drugs left behind.