It's an uneducated assumption made by passengers. There are no guarantees in commercial aviation. Even Rule 240 reaccomodations are at the behest of the airline. They are not legally obligated to do so since deregulation. It's simply a courtesy.
Now I will argue that it's good business to refund the fee should the bag not arrive as scheduled, but nowhere in the contract of carriage does it say that the bag will arrive or that the fee is to guarantee that it will arrive when the passenger arrives. It is for the printing of the tag and to offset the increased bag weights and fuel costs associated with carrying increased bag weights.