These "rewards" are rewards for the consumer if you: 1) Never carry a balance 2) Pay the same price as cash and the merchant eats the fees [as most merchants do!]
Its perfectly rational if you make fewer mistakes on average. So far, in the 10 years of using credit cards, I've recovered about 2% of every dollar I've ever spent and never carried a balance. This amounts to half a paycheck.
Its perfectly rational to assume that my rate of 'error' will remain roughly the same as it has been historically and that I want to continue to use credit cards and force merchants to effectively give me a larger discount that they would otherwise.