This is stupid. If an unloaded gun is in checked luggage, then it cannot go off by itself and nobody would be able to get to it while it's on the plane anyway. So other than some regulation mandating this, why does the gun need to be in a special case and airline employees told about it?
Because your buddy who works as a baggage handler on the tarmac loading the plane knows to grab the towel in the purple suitcase, then pass it to another “passenger” on another flight in the terminal who has already passed through security.