Is it legal to charge a fee for using a debit card?

Is it really legal for businesses to charge customers a fee for using a debit card? I've noticed some places adding a surcharge on transactions made with debit cards, and it seems a bit unfair. After all, debit cards are a convenient way to pay, and customers already have to pay fees to their banks for using them. So, is this practice actually allowed, or is it a form of hidden pricing? I'd like to know the legal stance on this matter before making any decisions about where to shop.