Maybe. The Consumerist has a bit on this, and they are trying to track down the actual rules.

We wanted to get clarification on whether merchants are allowed to say you must purchase a minimum amount in order to use a credit card. We called up VISA. The CSR said that it was definitely illegal and in violation.

. . .

We then called Washington Mutual. A tier 1 rep, after checking with her supervisor and credit card services, said that stores are allowed to require a minimum charge to cover the transaction fee they have to incur.

More details and an update contradicting that last paragraph in the full article.

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