> What do I mean by that? In an ordinary free market--let's take potatoes, beef, anything you want--there are thousands of importers and exporters. Anybody can go into the business. But it's very hard for a small person to go into the drug importing business because our interdiction efforts essentially make it enormously costly. So, the only people who can survive in that business are these large Medellin cartel kind of people who have enough money so they can have fleets of airplanes, so they can have sophisticated methods, and so on.

In addition to which, by keeping goods out and by arresting, let's say, local marijuana growers, the government keeps the price of these products high. What more could a monopolist want? He's got a government who makes it very hard for all his competitors and who keeps the price of his products high. It's absolutely heaven.

Source, in case someone is interested in reading more: http://www.druglibrary.org/schaffer/misc/friedm1.htm