It is crucial here that the resource being exploited is free and infinitely available outside the store.
Exploiting this free and infinitely available resource can result in forgoing a sale at the store, but calling forgoing a sale stealing is a logical error. A simple counter example: suppose I actually do steal an item and, as a result, forgo buying the item from the store. If forgoing a sale is stealing, then it would mean I actually stole two items! That is clearly incorrect unless 1 = 2, so forgoing a sale is not stealing.