You are generally want to have corridors so that you know where the drones are going to be, so you wouldn't just unrestrict everything, you'd have specific paths that the drones need to take to and from their destinations, like roads on the ground.

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.