As a I recall, the whole doctored candy scare was a myth. There are zero incidents of anybody getting a Razor Blade Apple, or whatnot.
And I do agree with you that regardless of correct terminology for negative/postive reinforcement, that handing out letters to fat kids instead of candy will very likely make the fat kid feel worse, not get better.
There are close to ZERO fat kids on the planet who don't know they are fat. Some stranger stopping to tell them, "hey you're fat, you shouldn't eat more candy" is not likely to boost their self esteem and cause them to alter their life trajectory in a good way.
I got no clue how you get a person to change their lifestyle, but I am pretty sure ruining a fun time for them is not on the list of best practices. Especially when fat people generally have a lower self esteem already (due to jerkholes singling them out about their weight in the past).