The credit card companies normally charges the merchants 3-5% over the transaction cost.
If I use a credit card which gives me cash-back, and I pay off the card balance every month, why should I use cash instead?
Not only that but if you're financially responsible, you can use a CC as an interest free loan. Citi just gave me a 0% loan for 18 months. The catch of course is that you need to pay the balance in full before then but that's why I say you need to be financially responsible.