While I have many gripes about airline pricing, I am not sure I get the logic of the plaintiff here. Many of the lowest fare buckets require round trip itineraries to qualify for the fare. To use these lower fares to and from the connecting cities you would have to book two nested round trip journeys to make the entire itinerary work. Isn't that already forbidden under Delta's rules?
If all four individual segments could be purchased at low cost as a series of one ways (and the LCCs have caused this to be the case in many markets), then I think they would have something. But wouldn't you lose the service, if you wanted to call it that, that the airline provides at the connecting city? How could you check baggage? You would also lose protection in IRROP situations, etc.