It was bound to happen. In fact, I predicted it years ago: eventually, a U.S. airline would charge more to use a credit card when buying airfare.
European airlines, such as Ryanair, have been charging this fee for some time. But U.S.-based Allegiant Airlines, second only to Spirit Airlines for its fees, isn't exactly calling this a fee. Instead, as this screen grab shows, they're giving a discount for using a debit card. Same difference though: you'll pay more ($4 in this case) for using a credit card.
It may be only a matter of time before other airlines follow suit. They might give you a discount if you use the airline's co-branded credit card, for example, charging everyone else more.