It doesn't have anything to do with being offended or insulted. The issue is that it's extremely irresponsible for a videogame to be determining people's health, particularly since the Body Mass Index is widely acknowledged as a pretty poor system of comparison for many people. It's this kind of ill-informed amateur advice which leads to eating disorders. It is also not recommended for kids that young to go on a diet unless they're morbidly obese anyway. Frankly, this was a rather predictable outcome, and someone at Nintendo should have seen it coming and removed this "feature" in the name of being responsible.
Now none of that means that banning the game is justified, it's just a stupid and very irresponsible thing to do.
Now none of that means that banning the game is justified, it's just a stupid and very irresponsible thing to do.