Technically, anyplace that refuses to take the card due to the transaction being too small is in violation of their merchant agreement. (One of my clients is MasterCard.) I'm not sure there is much for them to gain by going after the store that won't take it for less than $X dollars, so it doesn't seem to be widely enforced.
Besides, member banks provide merchant services as well as card issuing services. MasterCard, Visa, Discover and Amex just provide networks where member banks can process transactions using that brand of card.
Just like car dealers are representatives of the car maker, banks are members of card networks. So just like it's incorrect to blame Chevy, Ford etc when you encounter a bad dealer, it's equally wrong to blame MasterCard, Visa etc when you encounter a bad bank or merchant services provider.
Besides, member banks provide merchant services as well as card issuing services. MasterCard, Visa, Discover and Amex just provide networks where member banks can process transactions using that brand of card.
Just like car dealers are representatives of the car maker, banks are members of card networks. So just like it's incorrect to blame Chevy, Ford etc when you encounter a bad dealer, it's equally wrong to blame MasterCard, Visa etc when you encounter a bad bank or merchant services provider.