He’s freaking Lord Sesshomaru, that’s why.

Well, Sesshomaru produces poison himself. My guess is that he isn't affected by the other guy's poison because it's weaker than the stuff he produces himself, so it doesn't even bother him. But i dunno, just an idea.

If you ever observe Fairy Tale in the opening they have general spoilers through the illustration, granted not all of them are overt/clear given proper context and now you have a irksome formula