It is hard to talk of the final balance before the game is out of EA. I am more concerned with a proper lore-based justification for such a feature, because I cannot find it in neither common sense nor mainstream fantasy stereotypes.
I'm surprised you cannot find it in mainstream fantasy stereotypes. It's all over the place, especially in games (both pen & paper and video games). It even has it's own TV Tropes page:
http://tvtropes.org/pmwiki/pmwiki.php/Main/ReviveKillsZombie.As for common sense, it makes sense in game design where you (1) have undead characters, (2) want to allow those characters to heal/repair themselves because the game is not designed around having a finite health pool that can't be replenished, (3) have limited types of "effect" in the game.
Being poisoned is thematically the opposite of being restored. Thought game-play wise any sort of damage over time effect could arguably be the opposite of healing. Would it bother you as much if it wasn't healing vs poison, but light vs dark or something else?
