Since when was it illegal to arm a civilian aircraft? I know that some warbird owners used to maintain live .50s in their Mustangs. There are also privately owned aircraft with hard points for mounting bombs.

The Founding Fathers allowed and encouraged the arming of ships. They almost certainly envisioned a right to such, as well as to arm carriages, build forts, etc.

Families on what was then the frontier often lived in armed block houses replete with swivel guns.

If the idea is that the government not have a monopoly on force, which it was and is, then the more armed aircraft, drones, cannons, etc the better.