They absolutely must accept a card presented for any amount. It’s part of the TOS with every major cc company. This is the bar trying to shirk the service fees on small transactions, but it flies directly in the face of what the card issuer tries to guarantee, “accepted everywhere”. If they find out a business is refusing their cards when they don’t hit a minimum, they will warn the business, and if they keep doing it, will no longer do business with them. That’s a big reason some bar are cash only, all the cc companies cut them off. (Also tax evasion, obviously)

Edit: welp, I just did a little research, and while that USED to be true, it’s now allowed as long as the minimum is no more than $10. As of 2010.

Everything always gets shittier.