First of all Worgen are not Furries (Yes I said it).
The Furrys are the Vulpera. Werewolf mythology and lore existed long before the rise of the Furrys.
Beings of Light and Shadow still does not change the Light and Shadow itself. Demons are creatures of the Fel but even though they have Fel in their flesh and blood they can still be harmed by Fel itself.
A Void/Shadow Naaru like Muuru or Maara the Dark Star can still harm other Naaru who are in their Light form.
So that being said. Unless Blizzard Entertainment retcons their own lore to try and Square that Circle.
You have to understand how things work. The Worgen are a CURSED people. This is connected to Druidism magics and nature itself. It can heal and restore but it can also harm and curse. So the Light will inevitably interact with the CURSE of the Worgen.
The Light has got power to cleanse remove and undo Curses yes. But should the light be able to remove the Curse of the Worgen then they are no longer Worgen.
The Night Elf Druids simply made it possible for Gilneans to live with the curse of the Worgen but to not undo the curse itself. Since it was Night Elf use or misuse of the Pack form that afflicted others with the curse of the Worgen in the first place.
Living with a curse does not mean that you are not cursed. The Forsaken had to learn or be taught to not only come terms with but to “live” with the curse of Undeath.
The problem is. That the Light will be coexisting with a curse within the same vessel. This is as silly as Void Elf Paladin. To have the Void and the Light coexisting is to undo established lore and foundations of the WoW universe. If Void and Light could coexist then why would Void Elves be driven away from Silvermoon by the Blood Elves, if not to protect the Sunwell from the Void?
It is an obvious answer. Simply game mechanics. Nothing more. Not being able to play your preferred class with a different race will upset some of the playerbase. Then they will argue that Nerzhul was a Priest so why can’t Orcs be allowed to be Priests?
You can not have a race be able to be ONLY Shadow Priests and not Discipline and Holy Priests.
Nerzhul is a Shadow Priest. Even as Warchief of the Horde the Nerzhul that became the soul of the Helm of Domination or the Lich King. He never embraced or used the Light.
Orcs as a race are a Shamanistic people. Always have been always will be. Following the Spirits being in tune with the Elements not necessarily devotion or any connection to the Light.
In the Orc heritage armour questline. You see through Drakka the wife of Thrall. For the Greenskin Orcs to return to their roots, Establish customs and traditions from the old world (that they blew up).
Far seers are/were spiritual leaders of the Orcish people. So the Greenskin Orcs had no leaders when the elements broke their ties and connections to the Orcs, Only generals and Warchiefs but no Spiritual leaders. Thrall himself the “world Shaman” lost his connection to the elements and had to regain it.
Now please try and address the points I am making and not just argue for the sake of arguing.
My guess is that those within the World of Warcraft writing team will offer zero explanation or retcons. But simply take the: because we said so, path and establish it as the new normal. Like Gnome Hunters - Draenai and Tauren Rogues etc…