Being allowed to check in a firearm is one thing but ALSO allowing ammunition for the firearm is another.
Flying with a gun (as checked baggage) is no big deal in much of the US. I've certainly done it. Makes sense for me, given the crime rate in some US cities and motels, and at some highway rest stops, etc.
As for letting me transport my gun(s), yet not bring ammo for them - that seems just silly.
I'd have to buy ammo upon arriving at my destination. (New business opportunity - 24-hour gun shops or ammo dealers very close to airports. Maybe even with a courtesy shuttle van).
Then I'd have to throw the new ammo away before checking in for the return (or next) leg of my trip. Good quality ammo isn't cheap these days for many firearms. And in California, they reportedly will now require a background check not just to buy a firearm but also for each ammunition purchase.
Then there is the need to properly dispose of ammo, while traveling, as I prepare to head to the airport at 4 or 5am. (Shall I leave the ammo in a trash can in the airport restroom, or in the waste basket back in my hotel room? Perhaps I could hand the ammo to TSA to dispose of while checking my gun for the flight -- but even assuming TSA wants to be burdened with that, it still means I'm bringing a gun and ammo to the airport.
Seems much safer to just keep the ammo securely locked in my gun case, in checked baggage, while I'm flying.
The greater danger, by far, in my opinion is not someone traveling with a properly checked firearm, but rather the geniuses who leave guns laying around -- unlocked, and loaded, with a round already chambered -- in their home or vehicle for very young children to find and use. As they do, in surprisingly large numbers.
Now that's something we can and should be upset about.