MARIJUANA grows like a weed. Therefore anyone could grow their own crop of MARIJUANA for use. There goes your tax it argument. Why buy it from the stores, where it will have less potancy, higher cost and taxes to pay when you can get around all of that by growing it yourself? You are assuming that people are pretty much idiots and could not realize this on their own. Unlike grain alchol and beer, which takes different processes to more potent, the same process is needed for really potent MARIJUANA as does lesser potent MARIJUANA. All you need is a seed, water sunshine and there you go! I think you underestimate the laziness of many pot smokers. Why grow it when you can get a month's supply or so for $30-50? And when the store has a great variety? Growing it could only get you 2-5 or so varieties at a time, when at a store you could theoretically get 500 or so different strains....not to mention that while growing is easy, growing well and drying well can easily be messed up.
You are assuming that the government would allow more than 1 strain to be purchaced.