Don't worry, I predict drone sniping will become a new favorite vandalism of the youth. For those on the more nefarious side, theft from drones will probably be common. Bring it down in your vicinity, take the payload.
Plenty of young people put good skills to use in stupid ways until they get older (and some never hit "until"). Sometimes it's how they develop those skills in the first place.
The big reason why I think this is likely is the same as why I think it's more likely with autonomous car package delivery. An employee is a wildcard in the equation, they may care above all reason about the crime, and that can be the difference between easy money and going to jail.
If I wanted to, I could just walk around and take packages off as many stoops as I wanted.
Why don't I? Because it is illegal.
Of course 'youths' are going to do dumb things but I am doubtful theft from drones are going to become illegal. Although that brings up the point of stealing packages using drones for anonymity.