Is there an obesity problem in our society? No doubt. Is the principal responsible for the welfare of her charges? Of course. (To the extent that in loco parentis standards apply.) Has she pulled the lanyard on a charge that is more likely to blow up her cannon than it is to impact the target? Obviously.
Years ago, many, many years ago, there was a student in our elementary school who brought her own silverware with which to eat the school lunch. A teacher (remember zeal outrunning judgment?) chose to make an issue of it, though obtuse as I am, I couldn't see the issue. The girl, of course, enlisted her parents in this tug-of-war.
The father explained to me that the tableware provided by the school tended to react in an unpleasant manner to the girl's dental work, hence his darling carried her own sterling. Now we were not all born with a silver spoon in our mouths, but the fact that this gentleman had another daughter a year younger than the girl in question who did not carry table service to school lent credibility to his claim, and I said it was not a problem. In essence, I apologized for another's handling of what should have been a non-issue. Principals get to do that.
Well. I say. People!
Pick your battles!
When I used the term "Justin Bieber lunchbox" I did not know it actually existed; but as a long-time observer of the societal condition, I was not surprised when I Googled that phrase to find that it does exist. You can find it that way, too, should you want to send your kid to school with one. (Provided of course your kid doesn't attend that school in Chicago.)