Kind of a confusing story. The guy was already rung up and heading out the door and then got asked to leave his gun outside? Seems to me that's exactly what he was doing anyway, leaving the store. The time to check your guns at the door would be before you enter.
I'm guessing he was CC but his concealment wasn't so good. Some places you can be fined for openly displaying a gun, even if your intent was to wear it concealed.
It is also possible that this guy wanted to make a fuss. He could have informed the person at the door that he was carrying...