Or drove off to chase the attacker's vehicle. But, you're right: more evidence that it wasn't a LEO who was in the parked SUV. Never checked on the downed, twitching attacker, and drove away leaving him in the gutter.
I join the opinion that this was a drug-deal (or similar) gone wrong, not a carjacking. Would explain the parked SUV waiting in a clearly sketchy and lightly inhabited part of town; the vehicle pulling in with doors opening before it's even stopped and discharging armed assaulters; at least one occupant of the parked SUV already having guns up and expecting trouble; and the volume of immediate fire and then leaving the injured in the dirt.