One commenter wrote: "Yes, lots of people will be handing out $20 gummy bears."
I've read elsewhere (sorry, my interest in research only goes so far--I'm not checking it for myself) that marijuana tastes dreadful; suggesting that it would take a fairly determined glutton to finish one when tastier things are in the bag. Processed oils--that might escape detection. If the price is as high as quoted, though...
We've been hearing warnings like these for decades. No question, there are some pretty vicious people out there. But given the ease of identification, the lurking parents, and the kid eye for creepy people, I'd think the odds are against getting tampered candy.
If somebody want to tamper with stuff and put it back in the stores--most candies are double-wrapped. It could be done, but it would be hard to do in a way that didn't arouse suspicion. Candy would be the least of our worries from terrorists like that.
UPDATE: Crud. It looks as though either the stuff in the store was cruddy or somebody tampered with it there.