You have to think of it a little like a bank. When you put money on the card, even if you are playing the MS game and will only keep it for a short period of time, target is holding your funds. They are probably holding tens of millions of dollars (if not hundreds of millions). They earn an investment return on those funds that may add up. Marco may be right, if they get tens of thousands of people to switch and they now store their money (for short periods of time, but repeatedly) with them, they may stand to make money.