I've been tossing this idea around and have almost come to believe that government exists to make illegal those things it cannot tax, regarding any significant exchange of money or property between people.

They can tax profit (income) through work, so working and earning a wage is not illegal.
They can tax the sale of a car between two people, through property taxes, sales taxes (enforced when you have to register).
They cannot enforce taxes on prostitution, thus that is illegal in most places, except where they tax it. (Nevada)
They cannot enforce taxes on sales of marijuana, since anyone can grow it.  So guess what?  Illegal.
Even gambling is illegal unless it can be taxed.  Prime example is Washington State's new law against gambling over the internet, even though Washington has many gambling setups of their own, which take far bigger cuts than most online internet gambling sites.  The problem?  Washington cannot enforce taxes on gambling done online in other countries or states.

Word. If government could tax drugs, you'd have a coke and weed stand on every corner. The government doesn't give a crap about your safety, they sell guns in Wal-Mart FFS. All they care about is money.

It is easy to make alcohol, yet the government makes money from taxing it.  It is easy to grow pot, and the government could make money from taxing it (even if the taxation would be troublesome to regulate).

I'm not dismissing your idea though, it's interesting.

hmm. An interesting idea. I'm not sure the marijuana one holds, though: people could just as easily grow tobacco, but they don't seem to. In fact, drugs in general let this argument down: governments deny themselves a vast revenue stream by illegalising various intoxicants that could be safely produced, used and taxed, given sufficient public knowledge and awareness.