According to this article on CNN.com:

Buying gasoline with a credit card could be hurting your local independent gas station owner – and you may have to pay for it.

That’s because credit cards charge merchant fees in the form of a percentage of sales – and those fees eat into the fixed per-gallon sum that gas retailers tack onto pump prices.

Some stations raise prices in order to keep profit steady.

It makes sense. All I know is that I’m glad that I now have shredded all my credit cards and have taking public transportation to work for a full year now. The timing seems to have worked out in my favor.