Punishing people for quitting by keeping them from playing isn't going to solve anything. Quitters are going to quit whether you penalize them for it or not.
The better question should be this: why isn't there an option to rejoin a game I disconnected from, intentionally or not? I can't tell you guys how many RP boosts I've had stolen away from me by unstable servers and lag spikes, but I know that I'd love to be able to reconnect to a WZ game my team was winning 600-250.
The fact that I can't, and yet I'm still penalized for that disconnect (with no way to avoid penalty if the servers shit their pants and boot me), is complete and utter bullshit. Hold the slot for like a minute or two so I can rejoin within that brief time frame. League of Legends popularized the quitting penalty in competitive games, and they've had the ability to rejoin games for years.
Moreover, if we can join a game in progress in Arena, I should be able to rejoin it if the server boots me there, too. And this "cumulative quitting" thing that tracks people over the course of several days, monitoring their quits? That's shitty too, and only serves to piss people off and make them not want to play the game.
Quitting is a problem, but the system as is doesn't help anyone, because it's not getting at the real issue: quitters aren't going to be stopped by a 30-minute ban. They're going to quit regardless.