First of all, I don't know how you know the value of the dipping sauce is less that the cheesy bread. When I worked at the pizza shop, our breadsticks were made of pizza dough cut into strips. We mixed the dough ourselves in the back of the store so it probably cost a couple cents of dough per order.

Second, buy on get one cheesy bread promotion is most likely in place to drive up sales on other products. They know that a lot of people like sauce with their cheesy bread so they are likely to sell more sauce if they offer free cheesy bread. Its like asking a grocery store if they would substitute margarine for your buy 1 get 1 free loaf of bread so you can have toast. The margarine is cheaper than the bread, so why wouldn't they let you do it???