Their official reason is their claim that the wording of their brochures, passenger contracts, etc., is clearer on the fuel surcharge possibility. They claim RCI's wording was too vague and that isn't Carnival's problem.
As to the real reason - well, that is open to whatever conjecture someone wants to use. One could say they are being cheap, greedy, <insert a word>.
My opinion is they are saying "We're Carnival, the biggest boy on the block, and we aren't going to roll over without a fight." Let's face it, in the cruise world Carnival is the lone superpower and they know it. Sort of like Wal-Mart.