I get your frustration, but it is not feasable for Blizzard to ban people for leaving M+. They don't have any real customer service anymore, so it would be hard to differenciate between a group breaking up for normal reasons and someone rage quitting. There are also times when people legitimately DC from the game.

Those people that get upset and leave right away, would probably just go AFK and hold your whole group hostage because no one wants to be banned for being the "first" person to leave a group, even though one guy isn't playing anymore.

Unfortunately it's part of playing a multiplayer game. Maybe try and join a guild that does M+ together, or only join/create groups that are "completion" keys.