This is true - you can earn miles in Turkish Air's Qualiflyer program on Delta flights, but not on Turkish Air's flights if you're a member of Delta's Sky Miles program. I can think of two possible reasons for this: the fact that SR / SN / OS started the Qualiflyer program and since they were partners with DL already, they continued the partnership. Turkish Air and the others joined later and most did not have existing relationships with Delta Air Lines. Also, DL may not want to cooperate with Turkish Air because they compete on the New York / Kennedy --> Istanbul route. Delta is soon to enter the Atlanta --> Istanbul market as well. You should also note that some of the other Qualiflyer airlines are in the same position - AOM of France, Lauda Air and Air Littoral - have DL as partners in their program, but not vice-versa. Hope this helps ...