You can travel with a firearms as long as it is locked and checked in as baggage. It's kind of like the "Traveling across state lines" rule for transporting firearms in a vehicle. The gun was not on him while he was on the plane, he picked it up with his baggage loaded it and then proceeded to open fire. Believe it or not there are many instances where people will travel with their guns in baggage, hunters, reenactors, body guards, cops, military, and so on. It is a good process but eventually you have to give the gun back to the person and trust that they don't do anything stupid.