Disruption of an airport is one thing. flying a drone into a prison with drugs and phones is another. but once someone decides that the carrier of choice for an explosive device is not a person or a vehicle, which could be stopped, searched or shot, then this problem will be far worse, and no amount of registration or fees will prevent that.
You can fly one from 5 miles away over the nearest stadium or large gathering.
Armed forces have been using unmanned, but 'equipped' drones, for years to get close to targets.
We all have driving licences, insurance and car tax, but that has never stopped a terrorist driving into a crowd of people.