DAL doesn't.

TPG owned and had a controlling 53% of Midwest. NWA, which became a Delta subsidiary with the merger, owned 47% of Midwest, but was restricted from operational contraol. That 47% stake was written off prior to the sale to RAH. The terms of the TPG deal required TPG to be made whole prior to any return for NWA. TPG sold 100% of the equity to RAH. As of July 31, NWA, now DAL owns none of Midwest.

Midwest places their code on Delta flights. Delta code is not placed on Midwest flights.

While the situation stinks for Midwest, it has and had nothing to do with Delta.