With Halloween just around the corner, it’s time for scary news reports that begin, “With Halloween just around the corner…” This genre of yellow journalism often features warnings about tainted trick-or-treat candy, a mythical menace that in recent years has gained credibility thanks to the popularity of  marijuana edibles in states where such products are legal.

Last year police in Denver, where state-licensed marijuana merchants had recently begun serving recreational consumers, told parents to be on the lookout for THC-tainted treats in their children’s candy bags. As usual, no actual cases of such surreptitious dosing were identified.

But fear of strangers with candy springs eternal. The fact that this threat so far has proven imaginary is not deterring reporters and law enforcement officials around the country from warning parents that harmless-looking treats might contain a mind-altering substance other than sugar—if not marijuana, then MDMA.

Often the pretext for such reports is a drug seizure that occurs within a month or so of Halloween, even...