Who pays for these miles?

[Eboy]

Here's the deal. I have a United Airlines VISA credit card with which I get one frequent flyer mile for one dollar purchased. I can go online to the US Treasury Department and buy, with the card, a US Savings bond paying about 4% interest, which I can sell later. The Treasury Department's purchase limit is $60,000 per year. For each dollar spent, I get a mile. If I pay the monthly credit card promptly, there is no interest charge. There is a $60 annual fee for the card, which seems small relative to the value of the miles.

I could buy $60,000 of bonds per year, get 60,000 miles (a couple of domestic round-trips), sell the bonds after one year and get my $60,000 back with interest, and start the cycle over again the next year.

I tried it with just a small amount of money and it works. It is like free money. Who is paying for these miles? United? VISA? The Treasury Department?