But the problem really is...
That nobody has "tested" what happens when a plane and a small drone attempt to occupy the same space. So, being ever so cautious and dodging legal fights, the response is to say "they can't be here". Given the conditions, this is the only answer, and probably the least expensive.
This same mentality makes us take off our shoes when going through security and to ban water bottles through security (my test: drink some of it). There are also bans on silly things like radios and cell phones on board commercial aircraft for similar reasons. Someone feels there is a risk (no matter how small), and banning things eliminates said risk.
While I can see that even a small drone MIGHT cause some damage, I think that someone should (Mythbusters has ended) actually find out if this is true. In any task, there is some risk (even sitting at a keyboard commenting for ElReg), but understand the risks and take proper precautions. But it should be incumbent upon those stating the risk to figure out the ACTUAL risk, not some supposed one.
Life goes on.