There's two types of codeshares
1)airline sell seats on OAL flight by using their own code. Ie DL1015 is really AF007.
2)airline has OAL airline simply operate their flight for them. Ie AF1716. AF1716 is an actual AF flight, it is just operated by AF using Cityjet plane and crew. Cityjet doesn't operate it's own flight anf AF sell seats on it.
#2 gets mileage because its truly an AF flight just using different colored aircraft and crew.
#1 only gets mileage if both the marketing carrier and actual operating carrier are both ST.
My guess is since it's how AF does it with Cityjet, that
#2 is how these flights are setup.