Surely it would be common sense to not allow live ammunition on domestic flights.
From an air safety perspective, I'm more concerned by electronic devices and their batteries. Small quantities of packaged small arms ammo is unlikely to become a serious safety hazard .
From a security perspective, there's not much to stop armed perpetrators from simply entering the terminal from the street side. This crime occurred outside the security perimeter, where most airports should be considered relatively soft targets. Perhaps similar to shopping malls in terms of access control, though usually superior in terms of police response.
I think the pertinent question might be how much security do we desire and what will we trade for it?