From the Huffington Post article:
"The crew also mistook a telephoto lens for a rocket-propelled grenade."
That's why they opened fire, and assuming their mistake was an honest one, they were right to do so. A guy walking around Baghdad with an AK isn't a big deal and doesn't mean he's an insurgent, but an RPG is a different story, it clearly marks out the guy carrying it as a fighter or insurgent who is involved in violence against US or Iraqi national army targets.
Firing on the van is more questionable, although if they thought that the group of guys they initially fired on were insurgents, then it wouldn't have been unreasonable for them to assume that the van was driven by insurgents too (insurgent groups in Iraq have often used vans and other civilian vehicles to deliver ammo to fighters, pick up casualties, etc.; e.g. battle of Fallujah). Whether engaging it under that assumption was an ROE violation seems unclear at this point.
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