First:
http://www.youtube.com/watch?v=sDqIBk2IkTU
Second
I don't think it's allowed in football?! What would you say about Starcraft when they type 'GG'? Would you keep playing until every little unit of yours is destroyed? What about Chess when the player resigns? Are you gonna keep playing until your opponent gets a checkmate on you? Stopping the match in Melee is no different than any of those! If anything, I think it shows respect because you know when you lost! Most "ragequits" I've seen is when they get edgehogged on the last stock! Nothing bad about that!
In both your examples games are ended early in order to save a SUBSTANTIAL amount of time. Completing a chess game when check mate is imminent could still take 10, 20, maybe even 30 minutes. Same deal in Starcraft. We aren't talking the <1.5 seconds max that is saved by a rage quit in Smash during the final KO. There is absolutely no time saving effort.
There is nothing wrong with quitting a match when you know you're about to die on the last stock. It's just like resigning in chess. A person knows he has no chance of winning, so out ot respect, he doesn't drag it out to the checkmate. In fact, sometimes it's considered disrespectful NOT to resign in hopeless positions.
Again, just pointed out the obvious difference. In chess, you may have to wait a very, very long time to complete a game an opponent has already lost. In Smash you have to wait seconds.
People do it to keep momentum. Lain always does it, and he's never mad. He always knows what his CP will be, he just wants to keep the mentality.
The loser already gets plenty of advantages, I don't see why he gets the ability to try to make one more at the end of the game.
What it really is is this: the loser of a game gets to decide what the winner gets to see. That is UNFAIR to the winner.
The solution though is simple I guess: just turn pause off before the start of each match.
Also, people still bringing up the time thing: if you watched any of the quit-outs at Pound, you likely saw the same person that quit out spend several minutes thinking of a counterpick. You would think in a game like Melee where there has been...8 years or so to figure out CP's, and the stage list is very narrow already, and the same MU's occur constantly (this includes player MU's), that you wouldn't need 5 minutes to figure out if PS or BF would be your best choice.