There is a tremendous demand for illegal narcotics which is subject to the well-known Law of Supply and Demand.

If the legal purchasers limit supply available on the more "open" market, the demand for illegal drugs will drive up the cost above that which the legal purchasers will pay.

Now, you may say, it would be illegal to sell opium to anyone other than the government.

It's illegal now. Doesn't seem to stop 'em.

The problem isn't with the suppliers. The problem is with the demand. The problem isn't in Afghanistan, the problem is in the US.

Because the Law works in reverse, also. As demand decreases, supply lessens. When there is no demand, there is very little supply.

See "Buggy whips", which, due to a decreased demand, have suffered a decreased supply. Not entirely gone, but far less than, say, 100 years ago.

Arf