It’s exactly the inverse square law.

You get a million times more power if you can put a coil 1mm away from the wire than if you hover 1 meter away.

It's actually an inverse first power law, assuming that the distance of the drone from the wire is much less than the length of the wire. The inverse square law applies only to a point source of electrical energy, to a sphere, or to an object whose largest dimension is much less than the distance to the drone.

Drones consume something like 100W to stay in the air (ballpark, of course), so they'd probably never charge if they had to hover.