Greenie, I can tell you with all certainty that "stepping on it" is financially driven. While there may be dealers out there that cut their product to cover their own use, they'd either have to be using an awful lot, or selling very little, for this to make a difference. It's the same reason irreputable barkeeps water down their drinks or irreputable burger chains don't use 100% beef or irreputable builders don't use high-quality steel/concrete in their construction.

This is one of the reasons why I believe that legalizing (not just decriminalizing) certain substances is a benefit to society. If things like pot and coke fall under the same manufacturing scrutiny as tobacco and alcohol, we'd see far fewer incidents of impurities causing health problems and death. It's not the perfect answer, but it's better than what we've got now, and I realize that there are many who would argue that making reefer or a line of coke as available as a 6-pack is the first step on the road to hell.