It turns out that requiring a minimum purchase amount when using a credit card is against Visa's and MasterCard's stated policy.  Specifically, Visa's policy says, "Always honor valid Visa cards, in your acceptance category, regardless of the dollar amount of the purchase.  Imposing minimum or maximum purchase amounts is a violation."  MasterCard's policy says, "A merchant must not require, or post signs indicating that it requires, a minimum or maximum transaction amount to accept a valid MasterCard card."  MasterCard has a website where you can report a merchant violation; Visa recommends contacting the company that issues your card.  (via Mental Floss)