Visa make money by charging a fee to their merchants. So when you pay for a $100 meal with your card, the restaurant get something like $96-$98 at the end of the month (they probably have to pay an annual fee as well). In this case it looks like the US Treasury is paying for the miles - how does that make you feel as a taxpayer?
Cash withdrawals on a credit card (which is pretty close to what you're describing) usually incur an explicit charge for the cardholder (2.5% or so) - I'm assuming that hasn't happened to you.
If I'm correct, United miles seem to cost about 2-3 cents. Can anyone tell us what the approx. value of an air-mile is? What would be the equivalent fare in $ for say 10,000 air-miles (I realise the answer is going to be highly variable depending on the exact circumstances)?