https://en.wikipedia.org/wiki/Space-based_solar_power#Microw...
Found it: "Reason"
> At the Earth's surface, a suggested microwave beam would have a maximum intensity at its center, of 23 mW/cm2 (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm2 outside the rectenna fenceline (the receiver's perimeter).[82] These compare with current United States Occupational Safety and Health Act (OSHA) workplace exposure limits for microwaves, which are 10 mW/cm2,[83] - the limit itself being expressed in voluntary terms and ruled unenforceable for Federal OSHA enforcement purposes.[citation needed] A beam of this intensity is therefore at its center, of a similar magnitude to current safe workplace levels, even for long term or indefinite exposure.
Either the power density is high enough that it roasts anything crossing the beam path, or it's low enough that it's not appreciably higher than regular old sunlight flux.