[BrianG]

It's not so much the voltage but rather how air radio systems work. Min throttle on an air radio is brake/reverse on a car radio. The posix (or whatever it is called) translates the signal from the receiver so the ESC is receiving the right signal from a car radio. You usually don't have brakes unless you use a servo.

Also, air ESCs don't seem to have the pulse or burst current rating that a car ESC does. Airplanes tend to have a higher average current draw with less bursts than a car. A car system usually has very high bursts with a quite small average current.