Why does that matter. Its what you see. Most drug dealers out there are bad. What about gangsters? Why do you think they all get shoot up? Becuase they sell drugs, and go on each others turf.
Most drug dealers don't tell you how to use them properly and what not to do. You people just have some lucky ones i guess.
Take cocaine for instence:
They could put in powerd sugar in the cocaine so the could sell less cocaine in a baggie or what ever it comes in for the same price. Do you think he will tell you its tainted? No.