Are Minimum Purchases for Credit Cards Legal?

The other day I went to the hardware store and picked out $4.52 in merchandise. I went to pay with a Visa card and the following conversation occurred:

Clerk 1: "We have a $5 minimum purchase requirement for credit cards."

Me: "Oh... I don't think that's legal."

Clerk 2 to Clerk 1: "Just run it through."

Even though I technically "won" the argument, I instantly realized that I had NO idea if what I said was true! So I decided to look it up. :)

Here's what the CreditInfocenter had to say about it:

[Is it valid for a merchant to impose a minimum purchase?]

Never for Visa and MasterCard; generally not for American Express. Discover explicitly allows the merchant to set a minimum purchase amount, according to email received by the previous editor.

MC and Visa rules provide that a merchant may not require any minimum purchase amount. This is the merchant's agreement with Visa or MasterCard; it is not a Federal law.

According to Bankcard Holders of America, if a merchant takes American Express and also Visa or MC, Amex doesn't let the merchant impose a minimum purchase on Amex users because that would discriminate against them. Merchants who take Amex but neither Visa nor MC may impose minimum charges but Amex officially discourages the practice.

So in the above scenario I was right! Good thing I didn't try to pull that with my Discover Card!