Do we have any actual evidence it was a drone of any variety yet? I don't know how fast those planes are going at that point in the flight cycle but I would struggle to identify a "drone" at high speed, especially if I was concentrating on something else, say, landing a plane.
I think the current hysteria around these things may well lead to people mistakenly identifying things as a drone when it actual fact, they could be something like those stupid helium filled foil balloons you can buy.
It's impossible to identify a who is flying a drone unless you actually catch them at it at the time. Especially if it is thousands of little pieces spread over a large area after being hit by a plane. This is why people are so cocky about it at the moment, they know there is little chance of recrimination unless they are stupid enough to post it in a video on youtube.
Whilst regulation is great and the US system should work in principle, they have to register guns and there are definitely a lot of unregistered ones of the those kicking around stateside. You won't stop someone getting hold of the parts to build one (a drone) if they want to.
GPS geofencing is pointless as you don't need GPS to fly a Multi (or a plane for that matter)
As for terrorist functionality, you would need a fairly large multirotor to carry "a couple of pounds of explosives" and the flight time would be relatively short as a result. DJI s1000 size at least probably. You'd be better off with an RC Plane - much better payload and efficiency.
To be honest, I'm very surprised that the technology hasn't already been used in some sort of terrorist operation - RC planes with good payloads and great video/control range have been around longer than multirotors.
Kind of makes me wonder if threats to our national security are exaggerated for some purpose or another.