Small drone aircraft as a terrorist weapon?

I'm growing more and more concerned about the increasing number of near-misses between commercial airliners and private, so-called 'hobby' drones - small unmanned aerial vehicles that can be bought freely and operated by anyone with a smartphone.  Bloomberg reported today that there have been three recent sightings near JFK Airport in New York, and a quick Internet search turns up many reports of near-miss encounters between airliners and drones.

Simply forbidding the use of drones by civilians is a non-starter - there are too many of them already in private hands for that to work.  They're freely available at very low cost, too:  for example, Amazon.com has an entire 'drone store' listing dozens of models, many selling for well under $100.  Sure, the smaller models probably aren't capable of flying very high or very fast, and may not be big enough to cause serious damage to a big jet if they hit it;  but even minor damage will cause the air travel industry to throw a mega-fit.  It might also be very costly for airlines if an engine ingests a drone, because that will probably mean expensive repairs.  A turbofan engine as found on most modern airliners costs anywhere from $2 million to $15 million, depending on size and power, and spares and repairs are priced accordingly.

What worries me most is that terrorists can't be blind to the possibilities of this technology.  They must surely have among their members and sympathizers many individuals capable of controlling these small drones in flight.  What if they deliberately began launching them into the approach and landing patterns of major airports, seeking to cause a collision?  Even worse, what if they succeeded in seriously damaging or even destroying an airliner?  Can you imagine the panic among air travelers?  It'd shut down US air travel for a much longer period than 9/11, because there are literally thousands of these things out there, and no-one could be sure when one might not be launched from cover such as a clump of trees, or a city rooftop high above traffic, or something like that.  There'd be no way to trace it back to its launch point or locate the person controlling it.

As an avenue of attack, this looks to be both ridiculously simple and potentially catastrophic in its effects - and, from a terrorist perspective, turning drones against the USA in this way would be a very appropriate 'payback' for US drone airstrikes against them in the Middle East.  For that matter, some freely-available larger drones even advertise their cargo capacity (see, for example, this Volantex unit).  What if a terrorist were to load one up, not with a camera and batteries, but with a couple of pounds of Semtex and a contact detonator?  That way it could be directed against ground targets, not just against airliners;  and if launched at night it'd be as near to invisible as makes no difference, preventing many countermeasures from being effective.  What if one were to hit the windows of an airport control tower, or a fuel truck refilling a gas station's tanks, or a critical component of an electrical substation?

We've already seen Hezbollah and Hamas use drones in the Middle East, both armed and unarmed, ranging from small hobbyist models to professional military types.  What's to stop other terrorist groups using the same technology here in the USA, where it can be bought over the counter or by mail order?  I can't see any easy answers to this threat.  What say you, readers?  Any ideas?

Peter