Why would magically animated undead like zombies be turned by a holy cleric?

[b-dog]

I am just sort of wondering why magically animated undead like zombies would be turned by a holy cleric. It would seem to me that the process of creatig a zombie would not not be dependent on moral forces and it could be animated as an automaton just as a wizard could animate a suit of armor. The magic spell would just provide the energy to move this zombie around. Is there more to the spell than this? Does the spell invoke the soul of the deceased to some degree? Or are their darker forces that the necromancer taps into? Just curious, thanks.