Having dodged a few bullets in my time working at a gun shop, I can see where they are coming from. As a service center, I would assume they are dealing mostly with fixing guns and adding accessories, general gunsmithing stuff. In that case everyone coming in is likely to be showing them a gun they want worked on or parts for. Do you really want people pulling loaded guns and manipulating them in in front of you, especially one needing work done to it? Remember, a CCW does not always require training and I've seen gun owners who are doing good to manage to load and unload a revolver without shooting themselves.
Our solution was to simply post a sign saying all loaded guns must remain holstered. That let people come in armed but upped the safety aspect as they weren't whipping them out to show them to the gunsmith or whatever. If they had to remove the gun to show it, it was done under supervision and handed over to the gunsmith with no manipulation on the owner's part. Still, I've had a customer "unloading" a .45 Glock blow a chunk out of the concrete inches from my foot, numerous holes in the ceiling, and one "trick shot" that took out a wall phone AND a shooting trophy!