I can't. It's how it's applied since I've been here.

This is from the SRH which is effectively contractual. "A pilot will receive additional pay for any leg added to a rotation that could have been built into a rotation that would allow a pilot to report after 14 hours and be legal for the leg. If the leg was from a rotation that was run through the steps of trip coverage (outlined in Section 23 N. and O.), no additional pay is due. Deadhead legs qualify for payment if they are outside of 14 hours and are caused by a leg that came from open time. If the DH is to return the pilot to his rotation, no additional pay is due. Once rerouted, subsequent reroutes don’t restart the clock. You are paid reroute pay until you return to your original rotation."

The key words are "built into a rotation and be legal." If it is built into a rotation, it needs to start at a domicile.