Well, yeah. Anyone can own a small plane if they have the money. There’s plenty of uncontrolled airspace and uncontrolled airports.

Good God! What would happen if someone rented a box truck and bought some fertilizer?

Oh, and civilians can own muzzle-loading black powder cannons. Imagine what someone could do with a 32-pound cannonball.

The reality is anyone with the proper skills can crash a plane into anything they like. Unless you have someone on the roof with a MPAD, no one is going be to stop them in time.