The guy dropped a gun. That in itself isn't negligence.
He had a responsibility to carry his firearm safely, and he was clearly negligent in carrying out that responsibility since he lost control of his weapon and discharged it in a crowded public space. "Ooopsies... my bad," doesn't cut it when you're dealing with deadly weapons. This loser count his blessings, because in this state he could have been looking at felony charges instead of a slap of a wrist.