Although it says there have actually been documented cases of sharp objects in trick-or-treat candy. However, the documented cases cited in the Snopes entry all seem to be cases where kids put sharp objects in their own candy to freak out their parents, or kids put sharp objects in their siblings' candy to freak out their siblings.
In other words, it's a prank, but not a random-stranger prank. And, given that they all seem pretty recent, they were obviously inspired by years of razor-blade-in-the-apple urban legends.
Speaking of weird fears, where would we be without a little paranoid craziness from outsider artist Jack Chick?
(I particularly love the kid-hating dark-haired woman in panel 8 -- evil people always laugh by saying "haw haw" in Chickworld.)
This particular tract is from 1986, when the Satanic Panic was on the rise.
So, even though 1983 was a little early for the peak trend, there were certainly Protestant churches that were already freaking out over alleged satanism associated with Halloween, and having "alternatives" like "harvest carnivals."