The manual says to. I believe it's so that if you turn it on you don't shoot someone down. The trainer box is only powered when the instructor box allows it and the transmit side doesn't get power when in training mode so you don't have to use a trainer box on the same freq. as the Receiver. The instructor however must be on the same Freq. I would say it's just for safety. This came from pg. 17 of the Tower 6XM manual which I believe is the same as the Futaba 6EXP.