Because excessive weight gain is not exclusively linked to eating candy.
My Mom had a thyroid problem, for instance, that caused her to gain a lot of weight. Both Jerry Lewis and Rush Limbaugh had weight gain linked to certain medications.
My personal weight gain was caused by overconsumption of calories and a corresponding lack of exercise, but candy had little to do with it. Part of it was because of my salt-induced hypertension. But candy? I like it and eat it, but its a rare thing- I'm just now getting to the (standard sized) Toblerone I got for Chrismas 2012...and that's my favorite candy bar. So far, I've eaten 1/3 of it.
So unless you know the kid, you don't know the problems.
Edit: plus, who is more likely to be effective at delivering the proper nutrition message, a "mean stranger" who embarrassed them or someone the kid knows & trusts?
Also, who is more likely to get TPed or pranked because of said message.
Yup. Not every fat kid is fat because they eat bad food or because they are lazy.
And the ones who are fat because of those "other" reasons they can't control seem to be doubly sensitive to jerkholes pointing out their condition and trying to tell them how to fix something they don't have any clue about. Such fat people I've met in that situation already feel bad for being stuck that way, and then they get a heaping dose of superficial judgement by others assuming they are lazy and gluttonous as well.
It is generally considered rude to inject your unasked for solution to a stranger's situation when you don't know the details of their problem and thus cannot verify your solution applies.
Unless you come from a culture where that italicized statement is not the societal norm, then that should be the end of the argument right there.
As such, a person who agrees with that rule should inherently get that "YOU DON"T KNOW WHY THE KID IS FAT, SO KEEP YOUR ADVICE TO YOURSELF."
It ain't rocket science, and anybody raised by in polite society should know it.
Everybody with a problem hates hearing from the peanut gallery on how to solve that problem with a solution that turns out to be based on incorrect assumptions.
Therefore confronting a stranger with potentially faulty advice is rude, antagonistic, quite possibly incorrect.