Having a "leave" in a Co-op match shouldn't result in a punishment.
Many people play co-op to test out champions or get a faster win of the day. Usually one or 2 players missing doesn't ruin the game for remaining players since that the person that did stay in the game will get more resources for himself/herself allowing their practice to be more enjoyable. If a player disconnects in co-op, a bot also becomes "afk" thus evening out both sides of the match. So adding leaver buster on a person that left a co-op match isn't a fair punishment as the person who left already doesn't recieve XP or IP for the time he did spend in the game and leaver buster should be for people who left games that punish teams harsher for afk players like normals or ranked.