Vengeance and Vyper, your arguments hinge on the premise that doing something in your own interest at the expense of others is bad. This premise is inconsistent with the manner in which you are applying it. Leaving a game early is self serving and does have a negative impact on the remaining players, but it is equally self serving to demand that another person spends their time with you when that person does not want to and this will also have a negative impact on that person. We are all always going to serve our own wants and needs before those of others. If you try to argue like you're above that, your arguments will always have inconsistencies in them like the one I just pointed out.
There's a clear moral difference, as those asking people not to rage quit as asking them to fufill a moral obligation. To use an analogy, someone asking a murderer not to kill them is putting their own welfare first, but it's not wrong for them to do so (not that rage quitting is anywhere as bad as murder, but the analogy is important).
When you hit the ready / find match button on the game, you're making a commitment to play for a full match. In emergencies this can be broken, because it is a recreational activity, but barring that, you should finish out a match. You might turn around and argue this isn't explicit, but I'd say since most people do not like it, and there are ways of rage quitters playing with other rage quitters so as to not inconvenience those of us who would actually like to play the bloody game, there is no justification.