That's a big problem with legion class orders in general. It homogenized a lot of the classes from a cultural standpoint, and mashed together class variations that should be pretty different. Elune is technically a being of the light, yes, but it's a separate faith entirely, and their magic is pretty much the exact same as the star/moon-based spells that druids have. On a side-note, I'm pretty sure troll priests were always supposed to be witch-doctors or the like.
That's not how it went down at all. Tauren priests and paladins came about along with the Sunwalker Tribe- a tribe founded shortly after wrath following a theological discussion about the imbalance in worship of the sun and moon. The gist of it is that the Tauren decided the influence of the night elves has caused worship to lean far too heavily in favour of the moon, and thus they formed their own group to dedicate their worship to the sun in order to create balance. Tauren priests (seers) and paladins (sunwalkers) are really just sun-worshippers, not worshippers of the light- though yes, their power is technically the exact same thing (though it wouldn't surprise me if the way in which they worship it changes the way it manifests. The sun being a giant ball of fire and all.)