That is what RCI is going to do - put fountains on their ships for use only by those who buy the soda package.
Soda is a revenue generator and if they included it the cost of the cruise would have to increase. Not everyone wants to pay more for their cruise just to get soda included. It used to be that way though, with pretty much all the cruise lines including soft drinks.
You paid for it with your Disney cruise fare. I just looked at 4 night cruises in November. Carnival starts at $184 per person, followed by Royal Caribbean at $199, NCL at $259, and Disney at $499 - 2 or more times as expensive as the others.