Wikipedia gives a pretty good summary of how one arrives at that conclusion. https://en.wikipedia.org/wiki/July_12,_2007,_Baghdad_airstri... --- the short form is that the laws around war crimes give broad leeway to soldiers in the field interpreting a situation on the ground, and from the point of view of the gunship, they may have believed they were firing on attackers attempting to reach RPGs in the vehicle. This is one of the tragedies of modern insurgency warfare... Since nobody's in uniform, how can one be sure that the person in civilian clothing crawling through the operation is trying to escape or going for a concealed weapon?

I'm personally inclined to agree with Mark Taylor that "...what this video shows is really a case that challenges whether the laws of war are strict enough."

Within the US, there is a similar broad cultural argument ongoing on whether laws protecting police officers err too far on the side of assuming the cops' in-the-field interpretation of what happened should guide how the law understands what happened.

The Apache gunner did not know that they were firing on a journalist, they had believed they were engaging a legal target and were doing so within given rules of engagement. Which is legal. It's a tragedy that they were mis-identified, but again, not a warcrime. Had they known they were non combatants and fired on them still, that would have been a warcrime.