Hmmm, a very good point there. It would be interesting to make a study of cannabis users and average income per annum from selling confiscated goods (sadly impossible as these sales are obviously not documented). It would of course turn out that much more pot is getting into the country and sold illegally than is being siezed by customs. But what would the ratios equal in terms of pure profit to be made on stolen drugs compared to the profit of taxing all users of cannabis? Surely, like you say, the black market is limited by home growers and competitive dealers, which keeps the price reasonably low. But would that not apply the same if pot was legal and taxed? What is to stop even more people growing pot and decreasing the demand even further if it were legal? What would be the new cost of enforcing dedicated growers and no one else?
To sustain the demand as high as possible it is smarter to keep the drug illegal. When a big drug bust happens the demand climbs higher still. It would be a tough calculation to make and a big gamble of profits, far safer to remain as it is and keep the system going as long as the 'free' money is being made from confiscated drugs.