As pointed out, when Delta sold him a round trip ticket, and then checked his firearm, they knew that checking the gun on the return flight would trigger a police investigation which could result in felony charges. Upon check in, Benedetto could have been made aware of the somewhat unimaginable fact that a lawful US citizen may not even possess his lawfully owned, locked and unloaded firearm in one of the united states.
At that point, Benedetto could have instead flown in and out of Philly, perhaps leaving his gun in PA, or shipping his firearm home to himself without consequence.
At the root of this issue, is that NY and NJ have a handgun ban for all non-residents. THAT is the court case I am waiting for. I expect it won't go well for them.