My guess is adding the use of a manual safety increases the training complexity, particularly when you're talking about dozens or hundreds of officers, compared to the simplicity, particularly under the stressful situation of an officer involved shooting, of just pulling the trigger. You don't want a cop to draw a handgun and forget to disengage the safety if the shooting starts, and you don't want one to forget to engage the safety before holstering. The first time I had to draw a handgun on someone I had already previously practiced and trained extensively with it, in stressful simulated conditions. I didn't forget to disengage it in a real life situation, in fact it wasn't even a conscious thing, more automatic, or rote.The only Sigs I own are hammer fired. I've never tried the striker fired models, but I was under the impression that the 320 was available with or without a manual safety. If that's true, why would a PD authorize pistols without safeties?