It would be pretty hard to use logic to define fantasy, but here goes.
I would agree that in a fantasy setting that I could see a goblin be a shaman and NOT a cleric, however in trying to balance everything out with the races we had, they just didn't make the cut. To the logic/fantasy bit: Goblins went to school and got a lot smarter, which naturally led them away from the back hills shaman redneck role and more into the studious cleric role. Maybe GLT should have been gym library tan