That would probably cut into their margin.
When I worked at a gas station I spent the time one overnight shift to figure out how much things cost in the fountain drinks using the venerable 44oz as my best example. Typically the margin on something like that is around 95% but if you really want to stick it to the vendor grab an extra straw and lid as those were the most expensive things in the whole setup, then the wax paper cup, ice, syrup, and finally water+CO2 (couldn't figure out how to separate these costs since I didn't know how much CO2 is in the soda). Ice kind may stand out as odd but it is something that requires constant making even if it isn't used so that is why it ended up costing more than the water and syrup, but the bulk of the cost is in the cup, straw, and lid. Also the marginal difference in cost between a 44oz and the small 16oz one was about 3 cents for the producer.