I conjecture that this deal was constructed between AA and BD under the expectation that interest rates would be higher. When interest rates are very low, as they are now, it can be an excellent deal for BD customers. If AA sold the miles to BD at a fixed price, BD presumably is doing quite well from this at the moment, since I would imagine a significant gap between what they are getting on their deposits and what they are paying for the miles and tiny amount of interest.

Another wrinkle is that people with large bank balances in BD might not quickly move the money out once interest rates rise and the deal becomes less favorable. After all, setting up the accounts takes a certain amount of effort. So BD can end up with some high-net-worth customers whom they would not have gotten otherwise, and this could open up some opportunities for them.