Well, company can quite reasonably specify such a thing. Just as they can pass "no smoking" rules that apply whilever you are being paid, not just on company property.
Sadly, what it means in reality is that your only recourse is one of two things - don't work for them or else take them to court on the grounds that they are endangering you as a matter of policy.
As for the specific incident:
15 rounds fired seems overkill to people who don't understand the realities of using a pistol for self defence. You keep firing till the target drops to the ground or otherwise ceases to be a threat (throws gun away, for example). As for the slur on 9mm, that is bollocks, plain and simple. No reason to suspect that he would have dropped any quicker - shot placement not calibre is the key factor in incapacitating someone. The .45 ACP is larger and so more likely to hit something important than a 9mm if shot placement is not perfect (and let's face it, placement is rarely perfect in practice for most people) but that is the only thing that would give it an edge. The rest depends not on calibre choice but such things as specific round load, accuracy of shots, mental state of the target and probably others as well. He could just as easily have been using a .45 ACP, ran out in half the time and still come away fine. Or he could have dropped him with one lucky shot. Or he could have fired all his rounds, not dropped the guy and the guy finally figured out his gun was unloaded, corrected that and fired at the pizza guy. We just don't know because none of those happened. It is NOT a simple '.45 ACP beats 9mm' situation. There are no situations like this in real life, it's always grey.
John