Ice turns a cup into a cheaper serving
A fountain drink is not all beverage concentrate. It is usually a mix of flavored syrup, carbonated water, and ice. Because ice occupies space, a cup packed with cubes holds less liquid than the same cup filled without them. That matters to a restaurant: syrup and other drink ingredients cost money, while ice is relatively inexpensive to make or buy.
The cup still looks full, so a generous mound of ice can help deliver the appearance of a large serving without filling the container entirely with soda. It is a small per-order saving, but fast-food businesses sell enough drinks for small differences in ingredient use to matter.
The tradeoff is temperature and dilution
Ice also keeps a drink cold and can make it more refreshing, especially when it is carried away from the restaurant. As the cubes melt, however, they dilute the drink. A cup with little ice may contain more soda at first but warm sooner; a cup with lots of ice stays colder longer while gradually becoming less concentrated.
Restaurants generally design fountain portions around a particular cup and serving routine, so a full cup of ice does not automatically mean a server is cheating a customer. Still, the economics are straightforward: ice fills visual space that would otherwise be occupied by a more costly drink. The result is a cold, full-looking cup—and less liquid soda in it than its size alone might suggest.
