Just as Draenei Draw directly upon the Naaru for power and the spiritual enlightenment to wield the Light, the Tauren look to their Sun God An'she for the power and spiritual balance to wield the Light.
"The Light" is drawn from within the persons spirit, no matter what religion, how and why the draw upon it is different to every race, realistically, every race can draw upon it's power.
Tauren do it under An'she
Trolls draw upon it using the positive side of the Loa spirits, using their light
Night Elves Draw upon the aspect of Elune as the full moon and her teachings of peace
Humans draw upon it through their own spirit, channeling the aspects of compassion, tenacity and respect
Forsaken do so exactly the same way as humans, drawing on the Light through the ideals of compassion, Tenacity and respect.
Draenei draw upon it from the ideals taught to them by the Naaru.
and Blood Elves once used to drain the light in an arcane way, now draw from the spiritual emenence of the new Sunwell.
Tauren do use the Light, but not the Light everyone else uses.
But your duty to Azeroth is not yet complete. More is demanded of you... a price the living cannot pay.
The Light =/= the Holy Light
I think Halazz said it best. "The Light" is just the most common name of a semi-abstract concept that we don't entirely understand, because lore-wise it is not fully understood. The ability to harness the power of light doesn't depend on a particular religion, and it certainly doesn't require that it be referred to by a certain name. There is nothing incorrect about the Tauren priests and paladins basing their power around the light of the sun and it's place in their culture.
WoW characters that need/deserve to get killed/punished/otherwise removed from the story: Tirion(dead now), Thrall, Malfurion, Sylvanas(soon?), Jaina, Tyrande