RE: Why don't all radios have this feature? Add on if you have one.
very good point that exeter_acres points out. to improve on this idea like spyder said you can put a mosfet chip to not turn the rf signal on if battery voltage is below a certain point. But for safety I'm sure they would be able to make it not shut off the rf signal if voltage drops below the cutoff voltage after being powered on. Instead of shutting off the signal, they could sound an alarm allowing for a safe landing. And to Farther improving on that idea, maybe they could make the radio go into a sleep mode (if you will) if the transmitter is turned on and a certain amount of time passes without any stick inputs.