Is this legal???

My friends works in a restaurant as a server. As most people know, every time someone runs a credit card to pay for their dinner, it costs the company a certain amount of money, depending on the credit card company (i.e., Visa is the cheapest, American Express is the most) to run the card. Well, what her work is doing is, if the customer leaves her a tip on the credit card as well, they charge HER the fee that he company should be paying, and take it right out of the tip that that person just left her on the credit card. I don't see how this could be legal, and I certainly dont see in any way how this is right. Does anyone know if there are legalities against this? Thanks in advance.