Justice is a fan-made global mod.

But don't the priestesses serve Arkath? And don't the druids serve Silanne? And don't the priests serve Elrath?

Certainly, and they are also highly valued within their factions, as you can see. But witches have a different role, and the special position of women in the Dungeon faction is very evident: remember the appearance of the Dungeon city, their Grail building (Mother of Night), and the previously mentioned names for witches - Matriarch/Mistress, so witches, in this specific hierarchy, occupy a higher position.