ORIGINAL: MinnFlyer
The reason for this is that when the receiver is ON, you want it to find a good signal.
If it doesn't, there's a small but real chance that it could find a stray signal that causes a servo to turn farther than it physically can and do damage to the servo or to the linkages.
Not really likely on a 2.4GHtz system. The receiver will scan to find it's bound GID. It will ignore anything else.