i used to be like all you other guys, until i saw a yellow mcdonalds (this was last year sometime). It contained glass shards or what looks like glass. It could not be crushed and was certainly not crystal. johnboy, myself, nickstar, and ghost all saw this, and im sure there were others too.
not everything is urban legend. take what he says with a grain of salt, ie. he is warning they may contain glass, so if you are going to buy one crush it first, rather than accuse him of lying.