Some people just arnt that bright when it comes to paying attention to what they are smoking.
I recall being at a guys place with a friend, and the other people were all tweakers. A glass pipe keeps getting passed around, and my friend keeps taking hits off of it. After his 4th or so hit I say "Hey, may want to take it easy with that", my friend asks why, I say "Because it's meth..."
He 100% honestly thought he was smoking weed. Mind you, we were pretty wrecked prior to getting there on a rather large cocktail of other drugs, but still - even the obvious can escape people at times.
It doesnt matter if common sense should of dictated what they were smoking was not right, the matter at hand is - it's going around, and some people just dont know any better to not smoke it.