I have a drone -- I don't know if his version works different, but the telemetry is relative to the drone's take-off. The drone does not have topographical data to determine actual altitude from ground obstacles!
Someone correct me if I'm wrong -- but all my tests with my drone flying over changes in terrain (say a hill a hundred feet high) changes the telemetry altitude by zero -- it's all relative to the home point.
Regardless the guy was a pretty good shot to take it down if they were just doing a fly-over, albeit since the drone was loitering first, he probably had a good slow start to work with.
IMO the land owner owns that airspace and the drone operator was flying over land he did not have permission to cross. If he was flying at 400 feet it would be debatable. Anything less, the drone-op is trespassing. Aside from the city ordinance against discharging firearms, the land owner should have every right to down the invasive craft.
This reversal of arrest here is just to set precedence so Amazon can trespass into everyone's property without paying for air right-of-way.