This is essentially no different from the old-fashioned rabating Travel agents used to do all the time.
I think in this case they "may" be able to get around the policy because it is a club and they are essentially paying a membership fee that legally makes them something like a part of the agency booking the cruise.
Or I could be talking out of my bum. I really don't know.
But if an agency is doing that it is not exactly rebating because the customer is paying the full price. With rebating the customer pays a discounted price and the travel agent eats the loss - evem before his commission is paid.
Rebating is a tough practice - because the agent is essentially paying for the cruisers' discount. That was one reason it was stopped, too cutthroat, especially for small businesses.