Urban legend.
It may have happened a few times, put it absolutely was not so prevalent that people needed to cancel bake sales. Despite what you think, most marijuana users are not stupid - neither are they sociopaths.
It defies logic to believe that someone would waste their perfectly good, and expensive, drugs, by giving them to people who do not want them.
I encountered LSD spiked punch and pot brownies at at college party (ca 1970) But everyone was warned (At least early in the evening). LSD is a truly dangerous drug and looking back, I cannot fathom anyone being so irresponsible. College kids...