the paladins go to the death kngihts aying "yo for what you fuckers did you have to empower my weapon" and to stop the conflict they agree, and empower ashbringer, also its only your character that is part of a faction, the other death knights are just that, ebon blade, most of the ebon blade is just death knights, no alliance or horde
thats been basically a thing since wrath, you were sent in as a ambassador of the ebon blade, the main death knight to empose the part