How it used to be…
For starters, there was never a credit card minimum purchase requirement. It was never illegal for a store to set a minimum. However, doing so was blatently against their merchant agreements with Visa, MasterCard, American Express and Discover – all of which said a minimum purchase amount was not allowed.
If a business violated that rule, a customer could call up Visa/MC/Amex/Discover and report them. However in the several years leading up to the reform, they were very lax in enforcing this rule when a complaint was received.
How it is now…
An amendment of the Dodd-Frank reform went into effect in September 2010 that made it legal for businesses to set a credit card minimum purchase amount. Here’s the law so you can see for yourself:
(3) LIMITATION ON RESTRICTIONS ON SETTING TRANSACTION MINIMUMS OR MAXIMUMS. (A) IN GENERAL.—A payment card network shall not, directly or through any agent, processor, or licensed member of the network, by contract, requirement, condition, penalty, or otherwise, inhibit the ability (i) of any person to set a minimum dollar value for the acceptance by that person of credit cards, to the extent that (I) such minimum dollar value does not differentiate between issuers or between payment card networks; and (II) such minimum dollar value does not exceed $10.00