I think statistically you'll always see a correlation between weed smokers and hard drug users because, let's face it, most people start with weed. (excluding alcohol, of course, because we all know it's perfectly safe because it's legal)
I completely agree that you cannot blame marijuana for people choosing to try other drugs. But it's hard to refute a fact like "90% of heroin addicts started with weed."
It doesn't obviously take into account how many people smoke weed and DON'T end up shooting herion...