Strangers don't poison kids' Halloween candy
That is, although nobody can prove that it's never happened, neither can anybody point to a single documented case of a person putting poison or drugs or razor blades or whatever inside Halloween candy and giving it out to kids. Not one! There have been two documented cases of kids dying from candy that was poisoned by their own family, however. So keep that in mind - you're in more danger taking candy from your own family than taking it from strangers on Halloween. If you want something realistic to worry about, worry about getting hit by cars.
I guess this myth just persists because, like most urban legends, something about the idea of it is so utterly gripping to the imagination that everybody is willing to convince themselves that they heard it from a friend of a friend or on the news some years ago or something that deadly tainted Halloween candy is totally a real thing that happens.