you're so right - i never noticed this about zombies!
but we're forgetting a key figure here... Undine, the Queen of Graal, who's the only NPC in Classic to be put in jail, which means she has a motive... Revenge! and i mean, come on, of course the Queen of Graal has to have magic. i think king Tristam has too many responsibilities to raise an army of the undead anyways....