Everybody at one time or another has an "OH NO!" second. You just forget, don't think or just bad luck.
When I was managing a gun shop, we had the same rule about all guns holstered. I have had a few times where someone brought in something to be repaired. Handed it to me without checking. More than a few times I've ejected a live round.
Problem is that when you get into a conversation about guns, they want to draw thiers to show off. Chances are that:
I've got one in the display case.
I own or have owned one. Or, I'd never own one unless it was a drop gun.
I've seen a multiple dozen before and really don't want to see another, thanks.
It's nothing personal, we just don't want to catch a stray round from an ND. If we do, we'll probably return fire. Last thing I want to do is have a shoot out at my gun shop. Remember, I have more guns than you do.