Law of Kicking
Kempo's mandate to kick low is based on logic. A roundhouse kick and spinning reverse crescent kick to the head maybe flashy and impressive, but such maneuvers take longer to execute because your legs has to travel farther. They also expose your groin to your opponent's kick.
Kicking high requires superior balance and focus, you should practice your leg techniques high, and deliver them low for self-defense. Kicking low to the legs, executing a "pillar attack" can break your opponent's balance and legs.
As I tell my students, when your opponent is on the ground is the best time to kick them in the head.