Delta has this policy too, and actually when it was explained to me, it made total sense. They probably aren't used to dealing with $1, as this is more to prevent someone from buying a super expensive non-refundable ticket, using the flight credit for most of the payment, and a pittance on their credit card, and then try to get the whole cost refunded to their credit card during the 24 hour refund window.

Basically it stops people from converting flight credits to cash (or credit card).