Jacob Sullum puts it succinctly:
The argument that better pot is worse pot has never made much sense to me. There's essentially no risk of a toxic THC dose; the main health hazard from smoking pot comes from inhaling the combustion products. Smoking less of the good stuff (which is what people tend to do) is, if anything, less hazardous than smoking more of the weak stuff.
But it's not the health risks the drug-warriors are trying to prevent. It's the pleasure and the relief from pain. That's what they have to stop.