The Constitutional also makes the President Commander in Chief. As such, he is responsible for defending the United States.
Consider an American Citizen flying an airliner inbound towards the White House who proclaims his intent to crash that airliner into that building. Do you believe the President could order that airliner shot down?
You’re being obstinate - of course he can respond to an imminent threat, just like I personally can respond with deadly force to a deadly threat to me or anyone else.
The administration’s claim is that they can drone Americans on American soil, who do not at the time pose any imminent threat to anyone, without trial.