I was flying SAN-DTW on NW recently. The flight attendent called for all devices to be turned off. Then about 1 minute later, she explicitly asked phones to be "turned off completely, not just airplane mode." Of course, that was during the pre-flight taxi. I'm not certain how the flight attendant would have reacted had someone used a phone in airplane mode during the flight.
Perhaps the flight attendants just don't want to worry about what device is in airplane mode versus in full transmit mode. They likely just require them to be OFF before the flight so there is no doubt.