First of all, how sanctimonious to get all horrified!
Second of all, so many of us here do it.
A lot of us aren't filling up in peak hour and are using servos when there's not much of a queue, so we're not leaving our kids in there with 5 people in front of us deciding which payment method to use.
I tend to use my local Mobil because the price is usually competitive and even if it's not the cheapest, I use it anyway for location AND, the clincher...the card swipe at the bowser
Anyone who does the 'how on earth can you live with yourself' caper can get off their high horse. There are far more tactful ways to suggest alternatives that don't have you feeling like crap.