This is a spy plane, trying to spy on a hostile foreign nation. Of course the plane had its transponder off and of course it was up to no good. Spy planes/drones, well spy. if it was broadcasting its location, Iran would have been tipped off they were being watched, and their radio signals possibly intercepted, and would have the opportunity to try to impose countermeasures against being spied on.
A plane flying over international waters without a transponder on is not a threat and is free to fly under international law. Shooting down a plane traveling in international airspace is an act of war; this is true if the plane had its transponder on or off.
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.
The bottom line here is it was an UAV designed solely for the purpose of surveillance which was shot down with no loss of human life. It's the equivalent to someone flying a drone with a camera over/near your property and filming you while you shoot it down with your shotgun.