A credit is not a refund.
All US airlines allow you to rebook non-refundable tickets if the price drops. The problem is you have to pay a change ticket fee, which averages $200 for a domestic flight. The only US airlines which don't charge a fee for changes are Southwest, JetBlue (their policy is to charge a fee, but they don't), and Alaska Airlines, if the change is made at least 60 days in advance.