Why people ban Illaoi in normal draft pick?
On ranked, it is completely justifiable and even encouraged since there are some people who simply cannot understand that you should never test champions in ranked games. In addition, you don't know how to play against the new champion, which may be a problem if the enemy actually figured how to play the new champion.
However, in normal draft... I honestly don't know why one would ban her. It is a normal game, you are not going to lose anything important if you lose that game. Even if someone plays Illaoi and feeds as if there is no tomorrow, this doesn't matter. In addition, we don't know if Illaoi is broken yet, so we can't say that she is ban worthy.