Cash price for gas?

I thought this was illegal in Massachusetts. I've been driving by BeeZee's on Rte 20 the past week or so and noticed this sign. You can't really see it too well in the picture but it has a separate price listed for cash or for credit card transactions. I thought there was a law in Massachusetts that gas stations couldn't charge different prices for cash or credit transactions.

I mean, in this market I can understand why they would want to, but still . . .