Rewards are transparent. Spend some money and before you know it, you get a $100 check or gas cards or amazon credit etc. What a headache!!! I hate it when people give me money for free!
For free? Really? No, people give you money, as they are building a great big database of your consuming-behavior. Not only what you buy, but when, where, in which quantities, what goes together with what, and so on.
That is the currency, that you do pay for these "perks".