As horrible as that story is, I don’t think that fears about poisoned candy go back to that alone. In the 1980s there were a number of real examples of the general American public being poisoned – for example the Chicago tylenol murders[0] and the salmonella attack by the Rajneesh (Osho) cult[1], Because the USA is such a low trust society in general, it’s only to be expected that people’s fears would settle on the candy given out by strangers.

[0] https://en.wikipedia.org/wiki/Chicago_Tylenol_murders

[1] https://en.wikipedia.org/wiki/1984_Rajneeshee_bioterror_atta...