I've been hearing these stories, in various forms, since I was a kid in the 60s.

Never did I ever have anyone give me acid tabs, needles in candy, or razor blades in apples.
My mother was adamant about us not eating anything we collected until AFTER she went through it all. She insisted that it was for our safety.
It took a couple of years for me to catch on, but that was only because she wanted to pick out all the stuff she liked.

No one is giving out their stash. That's just a load of crap.

Don't give that away

Agree! You gotta bogart that shit!

why would anyone put razor blades in ice cream or glass in candy

1. You ASSUME normal people wouldn't do this....look around, there is a whole bunch of crazy out there!
2. What kind of parent sends kids to stranger's houses nowadays?
   Even in tbe 50's & 60's, Nana only took us to friends & relatives houses!

Therefore, a non-issue unless you are an abused/neglected kid, who probably ain't getting a costume anyway, and has much bigger problems.

If the police had their way, everybody except them would be imprisoned in glass capsules, so they could watch our every move!

Scaremongering.