Purely speculative, and personal experience, Rules of Engagement differ between theatre but usually along the same lines of 'you fire when there is an imminent threat to life' (not property!). You can shoot someone taking aim, but you cannot shoot someone planting an IED.
Either they defended an attack and then went after the guys retreating (not now an imminent threat), or they spotted people planting devices and decided it was a imminent threat to life - even though technically it may have been - but within the rules it wasn't.
My best guess, I could be wrong.
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