Sacred Haven of Debauchery

They left, but paladins still remained in the order. As did the priests.

I suppose this is a consequence of the gameplay. I don't know the structure of the paladin order in Azeroth, but there are some inconsistencies there. Tirion Fordring was expelled from the order for saving an orc, repaying a debt of honor. At the same time, Commander of the Scarlet Crusade, Renault Mograine, killed his father and remained a paladin. It seems that to stop being a paladin, you need to be excommunicated from the Order, but there's simply no one to excommunicate the Scarlet Crusade. And the fact that the Scarlet Crusade could use paladin and priestly powers is also quite logical; it's not divine patronage, but simply a skill in controlling a specific type of magic.