Because there is no probability that we are going to end up in large scale armed conflict with our own citizens. Ask a simple question, get a simple answer.
Why? What if some of them are spying for foreign power or prepare sabotage or are paid up by foreign powers to incite political unrest or, God forbid, political change that might end up in unilateral nuclear disarmament that might end up with destruction of usa with nuclear weapons?
The fact is you can imagine scary enough scenario to justify one thing but can't imagine other scenario to justify another. Thing is ultimately you need to act on some principle and current principle in USA is, us citizens have right to privacy, other people have no right to privacy.
This is silly rule because it creates places like current NSA. If it wasn't ok to spy on foreign citizens there'd be no NSA and if it was ok to spy everybody then NSA would have way more oversight.
Only if NSA knows if they are. But what if they don't know yet? Don't you want them to know rather sooner than later? You could easily achieve that by spying on your citizens rather than on the rest of the humanity your citizens might be spying for.
The premise of the preceding question is that the US somehow goes easy on its own citizens when they do things that might justify electronic surveillance. But that's obviously not true; there's just a different process for it.
The fact is you can imagine scary enough scenario to justify one thing but can't imagine other scenario to justify another. Thing is ultimately you need to act on some principle and current principle in USA is, us citizens have right to privacy, other people have no right to privacy.
This is silly rule because it creates places like current NSA. If it wasn't ok to spy on foreign citizens there'd be no NSA and if it was ok to spy everybody then NSA would have way more oversight.