It's called journey control technology --
"Journey Control (Booking O&D)
Journey Control technology eliminates the ability to book one flight segment at a time (point-to-point) when more than one flight is necessary to get the passenger to his or her final destination. Creating a reservation that includes connections must now be booked from a schedules or availability display that includes the passenger’s complete origin and destination (O&D).
This technology will review every booking and itinerary change to ensure compliance with O&D guidelines. Bookings or cancellations which violate Journey Control rules will receive a unique error message denying the transaction.
Journey Control provides customers with consistent availability found across all distribution channels.
Note: Journey Control technology allows Delta Air Lines improved detection and enforcement efforts for ticketing abuses. Agents engaging in this practice are held accountable for these violations and appropriate action is taken in accordance with Delta's Abusive Ticketing policies."
from delta.com
"Ve haf vays of making you comply, Ja?"