They actually can *assign* them, if it is done right at the time of reservation. However they cannot *change* any seat assignment once one is in the PNR.

For Delta to be able to assign elite seats:
- reservation has to be booked with a live agent, it cannot be done on delta.com
- partner elite FF number needs to be in the reservation right from the start
- the Delta agent needs to request the seat right away when making the initial reservation (before the first transaction is ended and the PNR code is being generated)

Anything other than that will result in AF automatically assigning whatever seat the system automatically returns, and as said, once it is in there, there is pretty much no way for Delta to change it.

As chrissxb has said, if the reservation is made in Amadeus, then the seats can be assigned anytime and can also be changed.