All sorts of things could kill people in exactly the right circumstances. A handful of people have probably died after losing control of their vehicle swerving around someone's dog that wandered into the street. Just because something can add the final straw to a situation causing a bad thing to happen doesn't mean the likelihood is high enough to be worth regulation.

Considering how many drones are in operation ans how many accidents haven't happen I think we should err toward no license required under a certain altitude over private property (the same altitude at which you need to put a red light on a structure in that area sounds like a food starting point).