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.
Seems to me that in a place where mystery people attack oil tankers, some eyes in the sky are definitely in everyone's interest.
Oh, wait. Iran doesn't want those eyes in the sky, do they.
It is obvious who is behind the attacks on the oil tankers. Iran wants the world to know they are behind the attacks, while denying them strongly enough so other countries will not be willing to take military action because of the attacks.
Iran knows Trump does not want to go to war with Iran, avoiding pointless wars was part of Trumps platform that got him elected. I believe Iran was hoping to show the world they can attack the US without consequence, so they can show the world they can also attack civilian ships of other countries without consequence in order to persuade other countries to evade US sanctions against Iran.