It was in the parking lot..........
Well, if you're going to board a plane, why would you carry a hand gun into the terminal check in desk? The rules for traveling with firearms would require putting them in checked baggage with ammunition stored separately. There are lots of destinations that are hostile to visitors traveling into a state with any kind of firearm. It's a good bet that people parking in the airport parking lot are not going to carry any kind of weapon where it is easily retrievable. If you slip up and end up carrying a handgun into the screening area, there can be serious legal consequences. Likewise people flying back are also pretty unlikely to have a firearm easily accessible for self defense.