Actually as far as I know (I've been told this by a bank manager, so take it for what it is I guess) this offering a discount for cash prices is still against the merchant license agreement. Computer stores here in Edmonton have been doing it for years and if you threaten to call Visa/MC and complain they will waive the "fee" and give you the item at the same price. It's a violation of their agreement and they could have their license revoked and loose the ability to do credit card purchases. I believe this now also includes debit card purchases since they use the same hardware. Being "cash only" is almost suicide for most businesses...
I practice the policy of not giving my money to businesses who follow this cash discount practice.
From my understanding the agreement states that the price charged can not be different than than for any other purchase method. They have removed the word "fee" to avoid the above work around. I have not read any merchant agreements, but this is what I have been told. I have also discussed this in the past with a phone rep from Visa as well.
In any case bring this up with your local store owner and make it clear to them this is against the rules. Explain that continuing this practice could result in the loss of their merchant license and I think you'll see the discounted price now applies to credit cards as well.
As for the minimum purchase I seem to remember this being a clause in my credit card agreement back when I read such forms(I think it was $10). Then again that was back before you could pay for parking or rent a movie from a vending machine on the street...
I think I'm the opposite. If a company offers a cash discount, especially a Gas Station...I'm more likely to shop from them.
This is in part because of my budgeting to typically only carry cash in my pocket.