I am not sure if it is a law but the rules of carriage for the ability to accept a credit card, state you can not do this--that you are required to accept a card for any amount of purchase, if you accept the card at all. Contact your business manager at your credit card servicer for the exact details; but you can be reported to the card (ie VISA) and they can pull your right to accept that card as payment.
I don t believe that the business is violating any law if it is accepting credit cards only above $10. In fact, some businesses ask for a photo ID above a certain amount, and one might question the legality of this too. I think a business has the right to set its policy in this matter.In any case, unless you are the owner, you should approach your supervisor about this issue.