https://en.wikipedia.org/wiki/Free_refill#:~:text=Due%20to%20the%20extremely%20low,refills%20as%20a%20sales%20gimmick.

Due to the extremely low cost of fountain soft drinks (especially the beverage itself, not including the cost of the cup, lid and straw), often offering a profit margin of 80-82%, establishments tend to offer free refills as a sales gimmick.