It's no longer enough that they already scam you on the fraction of one cent that is on every fuel station sign, per gallon sold. Try this practice on anything outside of a fuel station and you'll find yourself explaining your actions to the authorities.
The new rage is to charge customers different prices, depending on the payment methods. This wasn't something that vendors accepting plastic were previously able to do.
The problem is that fuel stations have decided to suddenly switch the long accepted high visibility advertisement for the credit price and replace it with a cheaper but seldom used cash price. The most common medium for paying (plastic pricing) is now more expensive and is shown on much smaller sign that is difficult to see, usually placed on the ground or away from where people are used to looking.
Typically drivers notice their vehicle is about empty and promptly look for the best price per gallon while they drive down the road. When one sees the most competitive price they pull in for the big purchase. You may notice the two different prices at that point and drive off or or you may fuel up as I did at my local Shell station not aware of the price difference until after you see the receipt. Unlike other mistaken purchases everyone knows you aren't going to return the fuel you just payed too much for.
To me this practice is shady in the least and nothing short of deceptive. I hope people unite to avoid fuel stations all together that operate in this fashion. Unfortunately being aware of this issue isn't always enough, because it is such a sneaky practice.