So was the drone flying over their territory or not? I find it hard to believe that it was circumnavigating Iran just outside of it's borders when it was shot at.
Likely it was spying on them and got to the ocean before they could catch it and where they failed to shoot it down. So actually the the ones who committed the act of war in this case are the US, not Iran who only shot at a spy plane that had recently been over their territory.
Shooting spy planes down is not an act of war, but spying is.
You have a completely unethical view of this conflict.