[aetherson]

You use masers to beam the power down. The inverse square law is based on the idea that the power that you send is radiating out in all directions (thus, the power experienced at distance d from the radiator is based on the surface area of the sphere with radius d, thus dominated by a 1/d^2 relationship).

With masers, you aren't radiating out, you're pumping all the power in a straight line and you can capture approximately all of it by building a capture point as wide as the line.

(More technically, since of course you won't create perfectly parallel microwaves, there will eventually be dispersion, but it's not meaningful over the distances that you care about.)