LOL! I could have given a better explanation as well.
GMRS is a "channelized" service. As such the frequencies it's allowed to transmit on are hard coded into channels. When in Memory(channel) mode, you are locked down to transmitting on only the approved frequencies and are therefore allowed to transmit. In VFO mode, you have virtually infinite frequency adjustment in the band as Steve indicated. That is why you can only listen in with VFO mode but you can transmit in Memory mode.