It deals with a conflict between lore and game mechanics, this effects priests more than any other class.

Different races draw power from different sources, the majority of priests do indeed draw power from the Holy Light, humans, dwarves, draenei and probably blood elves, also likely gnomes in the expansion. However the other priests, night elves, trolls and forsaken, technically use other power sources. Night Elf priests draw power from Elune, Trolls from Loa spirits and the Forsaken from the Forgotten Shadow. Therefore lore wise Forsaken are shadow priests, there are instances of them still FOLLOWING the light, but by and large when you are undead the light itself is very difficult to call upon and harmful to you.

This is where game mechanics come in, they had to standardize the priest class for the game, since most priest races use the light that's the flavor Blizzard went with in terms of spell names and appearances. The Forsaken get their personal style of priesthood in the Shadow tree, that's what your "typical" Lore-wise Forsaken Priest should be like. Trolls and Night Elves? Well Elune and the Loa don't get much play.

Anyway to end a long winded post, since technically the Forsaken can no longer properly harness the Holy Light they cannot be Paladins.