Abstaining in One For All bypasses Karthus Instaban

I went into a 5 man queue with my friends and we all decided to press abstain to see what happened, we ended up with rengar (so we know it's a random champion). Then we did it again and somehow ended up with karthus, an instabanned champion in one for all. We didn't know what to do so we played the game through, here it is in the link above.