I think margins on gas are razor thin. That's why gas only stations are dying off, and are getting replaced by convenience stores, some of which stretch the definition being almost full restaurants and gift shops. They use the gas to get you there, and you buy other crap from them.
Anyway, the point is a credit service fee charged to the station can really eat into the profits, and they make even less than the small amount they were already making. It was standard practice years ago to have cash/credit price differences at gas stations. That died out for awhile, and I guess it's coming back. It's not as good for the consumer, but I think it's fair, and I can live with it.