People who are willing to blow them selves up is impossible to prevent, but that part doesn't even enter this conversation about RC planes.
The 911 scenario was easy to carry off, box cutters were not prohibited items at the time.
Heathens slicing up flight attendants may seem hideous to you and I, but to them it is a method...which we now recognize.
They obviously had a high expectation of success, ergo it was not improbable.
You can't say that in your scenario.
Are you active in the hobby? I know R/C pilots who could put a 2 lb “payload” on target at 3/4 mile with traditional “Line Of Sight” techniques. The local R/C field has banned “First Person View” flying. http://timpa.org/
Also remember this from about 2 years ago- http://www.foxnews.com/us/2011/09/29/could-model-airplanes-be-next-terrorist-weapon/
Its not improbable, it’s happening.