that was because as a routine, the Iranians were setting the transponders on the F-14 as civilian aircraft. The A300 was headed directly over the Vincennes, was warned to divert on all freq's, but still continued.
That is why the Vincennes engaged.
Revisionist history.
1)The Airbus was climbing away from the ship, not descending as the US Navy claimed.
2) The Vincennes never tried to contact the Airbus on the civilian ATC frequency, and the broadcast statements were vague, they referred to the aircraft's groundspeed, not Airspeed.
3) The Airbus was on a regular civilian flight route, on a properly filed flight plan.
4) Two other US Navy ships in the area at the time identified the aircraft as a civilian airliner.
5) The US Navy had issued a NOTAM warning aircraft to remain at least 5nm from their vessels, the Airbus was 11nm away at the time.
6) The Captain of another US Navy ship operating in the area at the time had observed the USS Vincennes Captain's behaviour as "aggressive", the captain was known for "picking a fight"
7) The ship was in Iranian territorial waters at the time.
8) The US government paid out money to the iranian victims, while never admitting culpability.
Iran Air Flight 655 - Wikipedia, the free encyclopedia
Now I'm not say the US Navy and the crew of the Vincennes knew they were targeting a civilian airliner, and made a deliberate decision to bring it down. In the fog of war mistakes happen, but it is clear the crew and especially the captain were negligent in their conduct at the time. It is also possible these rebels/militants/terrorists/freedom fighters (however you choose to describe them) in the eastern Ukraine could have had the same mindset (we're under attack) leading to the same negligent action. It was the Russian (or Ukrainian government, depending on your point of view) who created this war, and continue to finance it, arm the rebels and stoke the fires in the region. Just like the US government has been interfering in the Persian gulf and Iran since 1953.
The difference being now the governments of the world are calling this a terror act, and a crime worthy of severe punishment, and with the Vincennes the crew had medals pinned to their chest and George H W Bush said "he'd never apologise for the United States, no matter what the facts were"
Hypocrisy anyone?