You called exactly what happened. However, no airline will make a new booking for a date change. You should have cancelled the first ticket and made a new booking yourself.

You should blame Delta, not for keeping the old booking, but for agreeing to use the haraSSSSment parameters that caused them to turn a customer into a haraSSSSee.

HaraSSSSment should be 100% eliminated and those responsible for its use should be severely punished.