It is the only way they can assure that the money is returned specifically to the person or persons who bought the item in the first place. Not all cards belonging to a particular person have the same set of authorized users. There could be cases where the purchase was made by an authorized user of one card with one person who has primary responsibility and the other card has a different person with primary responsibility.

Let's say a child is an authorized user on both a mother's and father's card and the parents are divorced. If the child buys something on the mother's card and returns it and gets credit on the father's card, the store is, in essence, transferring the funds from the mother to the father.