Hmmmm seems to be a mixture of the button code safes and the magnetic card safes on Carnival.
I agree that the code safe is the better way to go. If they don't want you to use your sign and sail card (for whatever reason) that means you either have to carry another card around with you or leave it "hidden" in the cabin somewhere. If you carry it off the ship with you and it gets lost or stolen then you can't get back into the safe without a purser coming to your cabin to open the dang thing.
I can pick a four digit code and remember it much better than worrying about which card last locked the safe.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007