You don't have to bring your own cup or drag one around for fountain sodas. They will serve sodas in a glass.
I suppose getting the baby on the ship FREE isn't enough. You also don't have to pack a dozens of diapers, you can buy diapers at every port.
In fact, most things you can't buy aboard a ship can be bought ashore. Can you imagine every ship having a Wall Mart sized store, where would the passengers sleep?