Refund and exchange policies vary by retailer. Though some limitations apply, retailers can generally enact whatever policies they prefer as long as they clearly communicate them upfront through signage. The common policy for retailers that grant refunds is to issue the refund using the same method with which you paid. Thus, if you paid with a card, you typically get the return credited to your card.
Additional Return Policies
One reason retailers credit refunds to your card is to avoid people using purchases and returns as an indirect method of cash advancement. A card refund can take several days to process through your bank, and the retailer is out that money until the refund goes through. A retailer also loses out because it pays a fee to accept your credit card payment and doesn't get that back when refunding in cash. Credit card fraud is also a risk for the retailer. A retailer may get into a refund dispute with the card provider when it pays cash back and the card is subsequently reported stolen.
