It is an insane marketing speak thing. The contractual requirement isn't for no price differences. There may be a discount for using cash, but they can't word it as being charged more for using a card. The two might be equivalent in numerical terms, but it is the language that is important.

Ah, Apologies there were some changes to this recently (2013) which had slipped my mind (they mostly don't affect New England due to state-based restrictions - which is probably why I'd forgotten):

https://usa.visa.com/dam/VCOM/download/merchants/surcharge-c...