I want to make a point here in favor of disciplinary action from Gom.
This is unlike a professional football team that does not play their starters in a meaningless game because of a few key factors.
First the smaller, less significant points.
1.) injury - self explanatory
2.) Although the coach's decision is to sit the starters, the backups are often playing for their jobs, and teams have a legitimate claim that they want to evaluate the ability of reserves to play on the big stage, both to develop reserves and to decide which reserves to keep on the roster.
Third, the most important point.
A half hearted effort to win is different from an effort to expedite a loss.
A team that does not show their playbook is similar to a starcraft player hiding his best strategies, in this situation, the competitor will use less effective, vanilla strategies; this is common in sports. It is also common that when games are meaningless or when competitors are not in the best mental state, that they will play either frustrated or give very little effort; John McEnroe's tantrums on the tennis court come to mind and very much mirrors the "bad mannered" actions that some players will take - using foul language or not "gg-ing". Teams and individuals also commonly play with visibly less effort when they are in a frustrated or disappointed state of mind, this is seen countless times when players or teams do not hustle on the court or field.
However, the above examples are not analogous to what Naniwa did. This is because the action of boxing probes and A-moving at the professional Starcraft level can be viewed as an action of self sabotage - one that attempts to expedite the process of losing. The analogous action here would be if John McEnroe purposely hit the ball out of bounds on his serve and then stood there as he allowed his opponent to Ace him - in order to expedite the loss.
In American football it would be the equivalent of coming out and kneeling down every single snap for 40 seconds and punting the ball away so as to keep the play clock running and reduce the overall length of the competition and through self sabotage remove all drama and tension from the game.
This goes against the very spirit of competitive sports, it is not an action that should be tolerated much less condoned. If Starcraft wants to be taken seriously as a competitive sport, it needs to hold its players up to the standards of sportsmanship that are applied universally in the world of professional competition.
I am a foreign Protoss player myself, and to see a very talented foreign Protoss be eliminated from the highest seed of the most prestigious tournament is disappointing. However, as harmless as some may view the act of Naniwa, a lack of disciplinary action from GOM hurts the legitimacy of the Starcraft 2 tournament scene as a location where outstanding individuals with fan followings compete for the entertainment of thousands, perhaps one day, millions of people.
A professional athlete takes on more contractual responsibility as their list of sponsors, and subsequently, the pool of their fiduciary duties grow. Their image becomes more important in comparison to their play as they are not only competitors, but entertainers as the business model shifts to one that is based on advertisement revenue through viewership. The perception of competitive integrity is critical to the continual growth of the tournament scene, actions by players or tournaments that hurt this perception will ultimately hurt the scene's ability to establish itself as a legitimate professional competition. (I'd like to add: as odd as it sounds, the act of forfeit is less damaging to the perception of competition than playing the game with an intent to lose quickly, because by withdrawing from competition entirely, a player avoids the pregame ceremonies, casting, and official representations associated with the event. Opting not to compete is different from competing with a goal of losing as quickly as possible.)
Although one may argue that half hearted attempts to win is not conducive to healthy competition, the purposeful attempt to lose, is such an egregious breach of that competitive spirit, it is only fitting that the tournament or team reprimand Naniwa to some extent for this action.
I am not saying the ban is a punishment that fits the crime, so to speak, however, I think a lack of punishment would ultimately hurt Starcraft in its bid to be recognized as a legitimate sport.
I realize my post operates under the assumption that Naniwa attempted to lose as quickly as possible, I am not a professional Starcraft player, so I can only assume this from my limited anecdotal experience with the game.