Because you know, it would be nice if people who walked through the X-Ray machines with guns actually got stopped. I mean, if I’m not allowed to bring a freaking water bottle onto a plane, or a full-sized tube of toothpaste, for example, you’d think they might actually catch the guns at the X-Ray machines, right?
Sure, this guy was some French government official’s bodyguard, but he’s still not supposed to get onto a plane with a gun on his person. He “forgot” — stupid? yes, maybe — but he did go through a special security checkpoint, and he absolutely wasn’t supposed to get through with a gun. Yet he did, and nobody stopped him. And once he got on the plane, he (apologetically) alerted a flight attendant about it. (She ended up stowing the gun “somewhere safe,” apparently.)
That’s the risk about being steeped in officious and pointless formality: eventually, you start treating it like what it really is, and that’s when the real security is revealed to be full of holes. It’s like trying to act tough all the time: eventually, you’re going to yawn, and someone is going to catch you. Except in this case, the stakes are higher.