Ok here it goes......
Spent 4 years as a Cold Drink Account Manager with Coca-Cola and dealt with fountain.
There are 2 kinds of fountain systems...
Post mix(bag in box) which the syrup mixes with the local water supply right before it get dispensed which is more than likely what WDW has since it has a huge profit margin and Disney gets kick backs from Coke on a yearly basis as well.Normally it costs around 2 cents per ounce.Even with filters if the water supply is not all that great you still get a smell or taste.
Pre mix(4.75 gallon tanks) which is essentially a huge can of Coke-this is a ready made product in which the syrup is mixed with super pure water at the production plant and is normally used in areas where the water supply is bad.
These tanks generate HUGE profits for Coke but little for Disney is they used.
Next time you buy a 2.50 (20 oz) bottle of Coke just a thought.....Disney pays about 36 cents per bottle.