Taco bell manages to stay in business despite having thinner margins and having their soda fountain out in the store where people can get unlimited refills on demand.

I seriously doubt this is a cost saving measure. The labor of having a human handle every soda plus the extra glasses that need to be washed (usually they give you a clean glass rather than just refilling the glass you already used) would more that outweigh any savings from frustrated customers giving up and skipping a 2nd glass.