I have a hard time believing somebody would lace marijuana with heroine. Its simple economics. Heroine is worth money, why would they waste it by putting it on weed when that won't increase the value of the weed?
Other drugs being laced I think is more common. I have heard of cutting cocaine with all kinds of weird stuff to increase the street value.
Not saying marijuana is never laced, as kids do weird things sometimes, but I find it hard to imagine what possible motive a big time marijuana operation would have to put heroine in it. This sounds more like fear-mongering to me.
Even if this was a real worry, it would be more of an argument for decriminalization than against it, because with legalization comes regulation.