How is that any different from calling in a fake bomb threat?

It's different from "I'm going to fly a robot into an airplane". My point is that gok's summary of this threat is rather off the mark. As for gok's analogy to a bomb threat... enough has probably already been said about that.

I suppose a bomb is a threat whether the plane flies or not, while a drone is only a threat if the airport decides to fly planes despite knowing there are drones in the area.

Apply common sense: One threatens violence, the other threatens inconvenience. Duh. If a "fake bomb threat," were called, it would have no effect. Bomb threats have to be credible to have weight, which would also an act of terrorism, meant or not.

What, they are just making up rules for fun? If it shuts down the airport, it is because it poses a risk to planes. Otherwise they'd just ignore the drone.

They aren't trying to kill people, so they aren't colloquially terrorists, but they are purposefully introducing risk to the activity of ordinary people.

Giving warnings is more responsible than the alternatives, but it doesn't really excuse putting passengers at risk.