Much of this goes back to the days of ships radio officers who were placed on ships to provide an element of safety. Much of the time they had little to do and to pay for them they would pass messages on behalf of the public (passengers) for which a charge was made. It was very like the land based telegram service. It was not unreasonable to expect a degree of confidentiality regarding such messages and so the secrecy of communication came into place. None of these messages involved the operation of the ship, but that was also covered by the legislation.