Not only was the helicopter not under attack but shouldered cameras were mistaken for AK-47s and a telephoto lens for an RPG... even though they look little alike through the high-powered camera on the chopper. In the last sequence a family driving by in a van happens to stop to help a wounded man. They were themselves targeted in a clear case of pre-meditated murder.
WARNING: This is quite horrible to watch.
The photographers killed here were members of Reuters staff, doing a dangerous job in impossible situations. Ironically these Iraqi journalists have, with this video, delivered to the world more powerful images than they might ever have taken in life. Meanwhile the pilot and gunners are playing a video game complete with whoops of joy and expressions of accomplishment and enjoyment ("cool!").
This is what goes on each day the US military occupies a foreign country. I am truly sorry for the military personnel who must now live for the rest of the life with the knowledge that they, through a willingness to be used as tools by the State, have been made murderers.
See also: Iraqi family demands justice for US attack death.