I have read many accident reports involving scattered passengers, dead and alive, and never once heard of a single one being run over by a vehicle. They sprayed foam everywhere but on the small fire near the still-present engine. Yes, it is inexcusable.
An investigator told me that the same thing happened at the Sioux City DC10 crash (I've not been able to independently confirm).
For someone who wasn't there, you sound awfully high and mighty. There was a massive debris field where that girl was lying - likely including lots of clothing and such that would do a nice job of camouflaging a young girl lying amidst the debris. Especially since airport fire fighting vehicles are designed to contain the fire-fighters within a (relatively) safe interior compartment. These aren't the fire trucks that drive down the street with guys hanging off the side.
Also, according to the reports I've seen, they've not determined if the girl was still alive when she was run over.
Tragic - definitely. Inexcusable? That's a highly questionable call.