Well, what's next? Will they start shutting off TV signals after Midnight since some kids might stay up all night watching television? Will they start banning kids from going to the cinema during exam time because that's when they are supposed to be studying?
Excessive MMO playing is a SYMPTOM of a problem, not the cause of a problem. I know I'm just repeating Extra Credits, but they were right when they said that people play MMOs to get away from problems - the problems don't stem from the MMOs themselves. Korean society, like Chinese and Japanese societies, are high-pressure, stressful societies. I would know - I grew up under that system. It's great for helping your kids to succeed academically and in keeping crime low, but the side effect is that... well, a lot of us who grew up with Asian Parents (or in my case, Asian parent) are a little bit.... warped. And that's putting it mildly. We are constantly told that we have to get a good job. We are constantly told that our behaviour must be perfect. We are constantly told that even the slightest mistake can ruin our chances at "a good life". Don't get me wrong: my mother loved me unconditionally, but she sure as hell manipulated her children into believing that she wouldn't love us if we failed at school or we failed to be "good citizens". When you grow up in a very rigid, very orderly society and family, with a TREMENDOUS pressure to succeed and act "normally" (i.e act in a way to impress the "all-important" neighbours and family relatives) you start to get a little.... off centre. You start seeking escapism.
Get rid of MMO's, and kids find something else to immerse themselves in. This law does nothing to address the underlying cultural problems which drive these kids to seek this sort of escapism.