Why does the Highway 22 kit have a “keyed-on power feed” for electric fan power vs. a “battery feed”?

It is true that a lot of hot rod enthusiasts want to be able to have the fan running after the vehicle is turned off.  The thought is that the water temperature will be better controlled by doing so.  Actually, when the fan is running in this fashion, it is only cooling water in the radiator and has no effect on the water remaining in the engine.  At that point, you are just increasing the drain on your battery.