You're ignoring the fact these aren't tainted souls, nor undead, nor naturally inclined to be evil in any shape or form. Your soul is much like an average Descendant, except it's half damned you could say, THUS WHY IT HURTS, though it doesn't just obliterate an Abyss Knight. The pseudo undeath, even if it was "tainted" like how you'd put it would still combat Ascended magic because through taking a Shadow Soul the soul becomes "darkened", however it becomes saturated with stagnant lifeforces that combats against all deity magic, so even soul warping magics would have difficulty bypassing their heavily stagnant lifeforce, which being the heaviest thing of a knight would seep far into their visages.