Bill, this is simply not true.
No prohibitive legislation of any kind has any sizeable negative impact on demand for any substance. Nor does wide availability increase said demand. However, those factors do influence price and they do influence crime, especially crime associated with black market activity.
When I lived in the US, the state I lived in at the time(going back quite a few years) had strict laws against Cannabis (to take one example), and lsd, and cocaine and all the others on the Federal list. However, it was not hard to find Cannabis - I got it nearly every day for free just by going to a certain area around my school. Cocaine and lsd were a bit harder to find - but one of the reasons is - they were not quite as popular. Not everyone wanted to be coked up or tripping frequently, but a whole lot of people enjoyed being stoned. If the demand for cocaine or lsd was higher, then it would be met by supply by someone. The fact is - they simply were not as popular as something like Cannabis so that supply never exceeded what the market wanted.
In my area (and as far as I know in the US as well), prescription drugs (many of which are both dangerous and addictive), can be easily obtained (and for cheap prices) with little more than a visit to a doctor. I know people that use them, and I know many (like myself) that largely avoid them. How is it that their wide availability has not prompted myself and many I know from just popping them like candy whenever we feel like it?