I believe last year American Express agreed to pay Delta some decent amount of money ($500M comes to mind, but I'd hate to be misquoted) basically as an advance purchase of Skymiles. I think AMEX buys FF miles at a discount, then hopes to recoup it (and then some) thru annual fees and transaction fees from members who acquire or keep AMEX cards that award Delta Skymiles.
BofA could certainly agree to a larger advance purchase if HP and US were agreeable to it. The DL-AMEX deal was back when Delta was going thru an uncertain phase, so them getting that advance from AMEX was beneficial (I'm sure AMEX got a pretty good discount on the miles).
Jeff