They don't care if you leave once you're in game, what they're talking about is people backing out before you've dropped in because you don't get a loss for that. If you are in game and quit and receive a loss that's fine. Take the loss and start over or continue your journey. They will ban people cheating the system to get to the lighthouse unfairly. Please do your research before you complain. Also it's only a temporary ban from what I've heard, like a week that you can't do any crucible or use trials gear. People lag switching and DDOSing are perma-banned.