The whole point of a transponder is it allows identification of an aircraft, so by default any aircraft without an active transponder is a potential threat regardless of the airspace it occupies.
That is ridiculous. Any US military aircraft is a threat to Iran, because the US and Iran are enemies.
Planes can do what they wish in international airspace. Unless the aircraft is provoking another aircraft, creating the need for it to be defended, it is unacceptable to attack the aircraft under freedom of navigation laws.