Just like with marijuana, firearms, and a whole host of things that they unjustly want to control. Why make it outright illegal under law if you can use taxes to the same effect. When they first outlawed marijuana they didn't actually make it illegal. They forced people to buy a tax stamp in order to possess or cultivate it. The problem was that you needed to either cultivate or possess it first in order to get the stamp, thus incriminating yourself. It was later deemed unconstitutional for this very reason.