It isn't churlish, it's simple mathematics.
Two Carnival shareholders, one is entitled to 10% loyalty benefit discount the other isn't. Each receives £150 obc. One buys items that would have cost them £150, the other buys items that would have cost them £135.
How is that a loyalty benefit?
Also the pricing structure means that you are purchasing your spending money(obc) and without it the cost of the cruise would be lower. This obc is not a gift.