There are many reasons people, even serious collectors that are not frequent flyers simply forget to “declare” the contents in their checked luggage. Airlines with all the cutbacks to actual personnel and automated ticketing, the actual check-in of flyers more often in a rush for everyone to get through, the check-in process becomes compromised.
Check-in baggage go through a special service area, this may have been overlooked or never questioned by airline personnel or contracted baggage handler. TSA does a X-ray of all baggage regardless if it’s checked in or carryon.
Ignorance will not given a free pass.
The DOJ/DHS/TSA has a standard of what makes a device a firearm. Believe it or not, black powder revolvers are not defined as firearms but cannot be taken on board. If the item was non-operating then no harm-no foul. Many of air-soft items are flagged in check-in baggage processing. No “declared” card needed inside the carrier. So it becomes a matter of semantics between flyer and authority.
Yeah, this one made the news. There’s more to the backstory we may never know or need to know. Take it for what it’s worth - a jab from the anti-gunner representative.
TSA confiscates a lot of stuff everyday, most of it never makes the news… until it needs to.