I've Asked Them
I've been in several local gun shoppes in towns in which (and near which) I've lived over the last several years. Nearly always I manage to work the question into the conversation.
- "Why do you carry on the job?"
- "Why do you carry that pistol in the store?"
- "Do you always carry that pistol at work?"
- "What made you choose that sidearm for your carry?"
- . . . and variations on those questions.
The answers all included some variation on protecting themselves and the business.
Sometimes the answer would provide insight into a choice ("well, I carry a .357 because . . ." or, more often, "I carry a 1911 because . . ." but sometimes, "I prefer the .40 cal because . . .").
In one shop, both the owners carried concealed. After I'd been a customer of theirs for a couple of years, I once complained about the concealability of [xyz] pistol, the senior partner reached back and pulled a 5-inch 1911 from his SoB position, cleared it, and had me try various positions with it (standing, sitting forward, sitting back) to see how it was he could conceal the "big gun" without difficulty. I had until that day never seen either of them carry visibly.
Their answer to "why" was basically, "you never know when some idiot is gonna try something stupid; you just never know."
Some of the owners and/or staff had stories of "that day back in 19xx when that moron/pair of morons/whatever tried to hold up the place," generally accompanied by mirthless laughter and a head shake. "You just never know."
This, of course, made it a real curiosity for me when I attended the gun counters in sporting goods stores and found that
nobody was carrying. Made me squint.
So, in general, from the people who actually work in those places, in response to my "why do you do that" questions: "you just never know when some idiot will try something stupid."
He was not visibly armed, however. I saw his sign up for a few weeks afterwards, but then it was replaced by something else. 
