There is a big difference between User... being called fat and Wii Fit calling this girl fat, namely, intent.
To insult someone, you must have the intent of insulting them, act on this intent, and then the second party must be insulted. User...'s classmates/teacher were trying to insult him when they called him fat--their intent was to offend. That's not the case for Nintendo, clearly. Wii Fit is simply a tool made by Nintendo. If their explicit intent wasn't to insult the user, they can't be responsible for any offense taken. You can't fault the first party for the interpretation, or misinterpretation in this case, of the second party. If there was no intent for Nintendo to insult, then no insult was delivered, thus no insult should be received.
User... himself used the phrase "filthy n****r"--something that many people, even in the context that it was used in, would find offensive. Was he trying to offend people with this phrase? Or was he using [traditionally] inflammatory language to prove a point? In his context, I don't think he was being offensive, regardless of how someone else would take it--same thing with Wii Fit using the word "fat." If the intent isn't to insult, there was no insult.
Language must be looked at with intent--just like any other action. Look at the legal system--Murder, manslaughter, self defense--all three ways of killing someone--same outcome for the 'victim' but the intent of the defendant is different in all three. You might disagree with this but that's how the world is working these days. Wrong isn't judged based on the outcome or even the action itself, but the intent of the person doing the action.
Unless there is proof that Nintendo's intent was to insult someone, they've done no wrong.