The crux is that regular resurrection spells simply restore the body. Which the tadpole has integrated with, so it is revived as well. (Along with the fact that the tadpole doesn't necessarily die when the host does. Throughout the game you're constantly finding living tadpoles in dead bodies)
The exception is True Resurrection, which creates a brand new, intact, body and thus Gale and his True Resurrection scrolls should cure the tadpole infection (Also, Gale's orb situation)
RP-wise, cleric spells are not spells*. They are prayers fulfilled by deities. A deity is not an idiot, and would know the tadpole is there. Nothing drags a god, kicking and screaming, into resurrecting that irritant.
* If you go back far enough, there is no "magic" per se, in the modern sense. It's all prayers to the supernatural who then alter reality for you. Hence treating witches as evil. It's not because of the spells, it's because the spells are actuated wishes by evil entities.
