Dealers don't usually grow drugs, they get it from a supplier. So even the dealer doens't always know whats in it.
The supplier usually has a lab or in the case of marijuana grows it like a crop in anattic. This sort of marijuana gets laced etc either to make it more addictive or to add cheap material/ingrediants to make more money from it.
If you get it from someone who grows it for their own use, then it is usually 100% natural and good qaulity. If it was legal, you would get more poeple like this growing it and cut out drug dealers and suppliers. I didn't explian myself properly before though. In fact most people who wanted to take it would probably just grow a small amount themselves. Just like growing some herbs or vegetables.
Completely agree with your other comment about a crime needing a victim.