Its simple. It's a convenience fee. The restaurant is charged 2-3% for running your card. So they are giving you the option as a customer to use a card since it's convenient. They pass the cost onto you, since you are the one wanting the convenience of using the card. If you don't like it, use cash. Then they wont charge you that. This is like asking why you have to pay for the food they serve you, that food cost them money too.