"free-fire zone:
NOUN: A battle area or combat zone in which no restrictions are placed on the use of arms or explosives. "

Where is the idea that free fire zone is kill everything in the area. The way you are using the term implies that free-fire zones are special areas in which the Geneva convention are somehow suspended.

"Thirty-two years after the event, the Viet Nam war is again in the news, with the revelation by Bob Kerrey that a squad under his command killed unarmed, civilian women, children, and elders in a Mekong Delta village. Kerrey, a Medal of Honor winner who has long had hero’s status in the United States, said that during his time in Viet Nam, he was ignorant of the laws of war, and so believed that, on the night in question, he was giving a legitimate order."
...
Among all our sources, the concept of the "free-fire zone" aroused uneasiness, if not outright rebuke. "A declaration of a `free-fire zone’ does not mean a soldier has the right to kill an unarmed civilian," emphasized one military source. "In a free-fire zone, certain decisions do not need to be cleared with one’s superiors back at base. But in no wise, is the free-fire zone license to kill anything that moves." "
http://www.crimesofwar.org/expert/expert-bob.html

The way I understand Free Fire Zones is that there was a miltary effort to limit warfare in certain areas. Unlike say in WW II, where soldiers didn't need permission in some areas- or in other words all of Europe or any war theatre in prior wars were a Free Fire Zone. US Soldiers are suppose to know the rules of war and nothing should cause a soldier to violate US law as applies to war and US law includes the Geneva conventions and other international agreements.