When I drop a pin directly on the runway of the local GA airport (no controlled airspace), B4UFLY says "Clear for Takeoff". Would it really be legal to fly a drone directly over the runway‽ Would it be trespassing, if not a violation of aviation regulations? It's a municipal airport. There is a fence with a gate, and I am not authorized to go through it. I'm in the US.
You wouldn't be trespassing, in the same way that a light aircraft can fly over your house without trespassing.
I'm not sure of the rules about uncontrolled runways, but if any planes were trying to use the runway you could be endangering an aircraft, which is an offense that's been around a lot longer than the new drone laws.
What airport specifically? This map shows FAA drone flight restrictions areas.
You will find more information on where you can and cannot fly using these VFR charts.