Oh they get punished. It's just a different, and entirely automated system. LeaverBuster is quite good at catching people for this. You can go ahead and report them at the end of the match for good measure, but it's not even all that necessary. LeaverBuster is going to get them on it's own. And it's not a forgiving system for people who consistently leave. Especially for players who do so in ranked, they get no more than 1-2 matches before they hit the Low Priority Queue. It starts off with a 5 game, 5-minute LPQ. If they leave/afk again, it goes up to a 5 game, 10-minute LQP. If they do it again, it goes up to a 5 game, 20-minute LPQ. At that point, if they keep leaving, they will stay at the 20-minute LPQ for a couple times, and if I'm not mistaken, it can progress to a suspension if they keep doing it long enough.
But the thing about a LPQ punishment is that you never see it. There's no outward sign that the person has been punished. They don't get a restricted chat, or a period of suspension, so it's not something that you can just look at their account and say "Yep. They got what they deserved."
While they may have some issues with getting the FeederBuster system up and running, and there is always debate about whether the Instant Feedback System works, one thing that people know works for certain is the LeaverBuster system.