You really want to know? It's because Delta is actually paying for all the fuel that the DCI carriers burn and in an effort to control costs, they make the mainline crews do these things. So as the mainline crews were doing all these things behind a DCI airplane which is taxing with both engines and the APU running, or sitting at the gate with the APU running, comments were made to Flt Ops about the disparity. Long story short, "You know what" rolls downhill, and management found another stick to threaten the DCI carriers with. Don't get your costs inline by burning too much fuel, and they will use the numbers against you ala Mesa.

Not saying it's right, just that's the way it is.