Because gun stores are no different from other low level intelligence retail help.
As a retail worker myself (I'm a student, and the tanking economy kept me there), I'd thank people not to generalize like that.
After all, I've been treated worse by bankers, mechanics, and computer techs than I ever have by retail workers, and I
hope they know what they're talking about.
But let me tell you, even good workers get worn down pretty quick by retail, because for every one intelligent question that gets asked, they have to deal with ten people with suits on and their nose in the air, and fifty John Q. Durp-dee-Durr's. If anyone in retail still greets you with a smile and good service like I and many other hardworking Americans still do, they deserve an award--either a purple heart or an Emmy.
I'd just assume it's bad workers, in this particular case. But, what do I know? I'm a college student, graduated in the top five percent of my high school class, have placed in the ninety-ninth percentile of virtually every test I've taken since kindergarten, and design and machine parts and tools that run off of 800 PSI air--starting as blocks of steel and aluminum--in my spare time.
But I have a low-paying retail job, so obviously the time and effort I'm putting toward a mechanical engineering degree means absolutely nothing, because a few lazy kids and counter-jerks instantly bring everyone else to their level.
Air clear and general whining aside, specialty shops seem more prone to this than a lot of stores. I used to get this treatment in paintball shops, still get it in computer shops, and have had people in mechanics' shops pull their crap on me when I was picking up my mother's car
while wearing my uniform from the garage I worked in at the time. It's because these places will hire people like that, and don't correct them from it until they're loosing some major sales.
Thankfully, I've found a couple good shops in my area and have no qualms about letting the bad workers at the other ones know that they can go somewhere else until I need their opinions.
The best thing you can do is let them know they lost your business, and why. One person walking out with $1000 isn't a major deal, sometimes. When they hear the same thing from a dozen people with $500, things either change or the lights get shut off.
However, the one that's still open is everything that a gun store should be, so no major issues 
