It must be technically possible to prevent a drone from flying below a certain altitude. Probably one of the many features that will be added to drones in the coming years.
Other commercial flying objects are rigorously safety tested for their ability to remain in the air despite different mechanical failures. Without the same standards applied to drones, we are likely to continue to see near-misses or full accidents happen. (Note: I'm not weighing in for or against drone regulation)
I guess the issue at hand becomes cost, between licensing, insurance and then building devices that are higher quality as if they could endanger life, I suspect they'll be cheaper than planes and helicopters but quite a bit more costly.
The key phrase here is commercial. There are strict regulations on commercial aircraft, but there are no such requirements for home-built or kit built aircraft. (quick edit, there are regulations/requirements for all aircraft, but it's a much lower standard for home-builts than for commercial)
Many people think that there aren't many kit aircraft out there, but there's actually a huge number. Oshkosh is the largest airshow in the U.S. for experimental aircraft and there were over 10,000 experimental aircraft at the show last year. There's even more who didn't attend the show! For comparison, Delta Airlines operates about 800 aircraft.
Most RC'ers I know are very intentional about when and where they fly; they aren't strapping a GoPro to them and flying them all around the city randomly and presuming they have the right to do so.
But of course, such thing would never happen. (Except it just did.)