Thus, DL has already received the revenue from the AMEX miles sitting in your account. They don't just have to pay when they are redeemed for a ticket. After all, that is part of DL's equation -- the knowledge that some of these miles will expire or never be used, but they will have gotten the revenue for them from AMEX anyway.
This all depends on how the value is accounted for. Until the miles are spent they are a "liability for future air transportation", against the cash they recieve for the miles. When they are actually spent, they are recognized as revenue, so technically they do not see "profit" from the sale of miles until they are redeemed.
The price of seats is agreed upon between the carriers. AF prices seats at $x per TATL ticket in Y and $y in C. Delta may very well rather spend $x to put someone on AF (assuming AF makes seats available to DL at that bucket), rather than lose the revenue associated with the same seat on DL. DL may very well lose money on the sale of that seat by paying AF less than it was paid for the miles. But you really never know unles you have knowledge of the actual terms.