Actually this has absolutely nothing to do with Delta, it has to do with American Express. AmEx credits up to $200/primary cardholder/year for Platinum cardholders each year for charges to the airline of choice.
I'm glad I'm not the only one who gets Delta/AMEX mixed up. Whenever I call customer service about something re: miles or credits, I have a 50/50 chance of calling the right company and always get it wrong