Why not? Because the money you're saving is actually money you're paying anyway. Although it seems free to you, merchants have to pay a rate to the plastic-processing companies to accept your card. Usually between 3% and 10% of the purchase price, depending on various factors. And when merchant costs go up, the cost of the product they're selling goes up. So prices today, in the age of plastic, are inflated slightly more than they would otherwise be. Your reward points are just the card company sharing some of its gains with you, to encourage you to push their product. So here we have a tragedy of the commons - Individuals are better off defecting, so that they can partake in the rewards. But society in general is worse off as individuals defect, because now goods cost more than they otherwise would. Cheers!