[throw20102010]

In this case Iran denies that it lost a drone so it couldn't be an act of war. You have to admit to getting shot before retaliating.

Also, we have seen multiple manned aircraft shot down during the Syrian civil war from multiple countries, and so far neither Syria, Russia, nor Turkey are at war with each other. It would seem that what determines an act of war heavily depends on whether the victimized country wants to go to war in the first place.

Iran wants to speak aggressively, but they do not actually want to fight the US. They have seen neighboring Iraq get steamrolled multiple times by the US Army and are currently sandwiched between thousands of US troops in Iraq and Afghanistan, and most other countries in the region would be more than willing to allow the US to set up logistics paths through their countries. They are in a horrible position to fight the US.

Edit so I don't create another comment: One of the reasons (but not the only one) why it is more appropriate to shoot down a drone instead of a manned aircraft is that a drone is easily turned into a kamikaze attack. Manned aircraft are much less likely to turn a hull run (flying close to a ship) into an attack, and are mostly for intel collection. Manned aircraft typically need to get their tapes back to base before analysis can start, and most pilots won't willingly kill themselves. While it kind of sucks to let the enemy snap pictures of everything on the deck of your ship (and record radar frequencies and waveforms), you aren't in any real danger and probably isn't worth killing them. Most drones could radio back its intel and then fly right into the side of the ship before the crew can react. Because of this, ships need to keep drones further away, and if the drone doesn't respond to radio calls they really have no option but to down it.