That's where predefined corridors are important. Let drones fly over restricted airspace with some registration and along "air roads".
If we could all have landing pads like we have mail boxes, something with a fence, it would be much better.
You don't leave it free space because then you have all sorts of drones whizzing around in all sorts of random and uncontrolled directions and that creates a lot of clutter that will cause crashing.