The airlines can't have it both ways. They advertise codeshares and "partners" as a seamless way to get from Point A to Point B. They don't have to add their own flights on those routes. The partner airlines probably pay DL to be part of the DL frequent flyer programs. Then when something goes wrong, DL backs away and says, "Not our planes. Not our problem." A more tragic example was the crash of the SwissAir flight a few years back- they code-shared with DL, I believe. DL denied any responsibility.