Strange!
I have not needed my passport verified to board any domestic flight MSP-ORD/DTW/ATL/JFK when connecting to an international flight, and mind you have checked 2 bags all the way through.
Yes, at boarding at the gate before getting onto the aircraft for the international sector have needed to have passport verified.
(last travel internationally was April'14)
Again, unless this is a new regulation by Delta and/or TSA.