I think that shouldn't be permitted. (As in, be illegal for the card companies to impose or enforce.) On the other hand, most gas stations seem to post credit

I think that shouldn't be permitted. (As in, be illegal for the card companies to impose or enforce.) On the other hand, most gas stations seem to post credit vs cash prices, so they get away with it. What's the story there? Why don't more merchants do it? It's also contrary to card agreements for merchants to add an extra fee ("it's +0.50 if you use a credit card, is that OK?"), but that doesn't seem to be meaningfully enforced.