Actually visa says there can be a minimum....
"U.S. retailers may require a minimum purchase amount on credit card transactions. The minimum purchase amount must not exceed $10 and does not apply to transactions made with a debit card."
http://usa.visa.com/about_visa/ask_visa/index.html
And here is what MasterCard says...
"A merchant located in the U.S. region or a territory or possession of the United States in another region may set a minimum transaction amount to accept a MasterCard® card that accesses a credit account, as long as the minimum transaction amount does not exceed USD 10 (or any higher amount established by the Federal Reserve Board by regulation) and does not differentiate between issuers, or between MasterCard and other acceptance brands."
http://www.mastercard.us/merchants/support/transaction-amounts.html
They both brought their terms inline with the law back in 2010. Many people do no know that.