This is a workplace injury, no different from those that happen everyday where a safety mechanism is not engaged and something falls, someone gets electrocuted, etc.
The gun wrangler certifies the prop is ready to use, and that's it.
They are going after him for PR points and/or that's where the money is.
Ask yourself, do you perform a safety inspection every time you rent a car, checking oil, trans fluid, brake fluid, brake pad thickness, rotor thickness, and so on?
Then you would be guilty also by your standards and logic for a brake/equipment failure resulting in a death/injury.