Overregulation reifies the black market.

More broadly, the desire to control the marijuana market via regulation is misguided.

If regulation is mild, it has no meaningful effect. Consider a rule that limits purchases to one ounce per customer per month. For most users an ounce lasts at least a month anyway. And consumers who want more can purchase at multiple stores or have friends or family purchase for them

If regulation is instead strict, it promotes continuation of the black market. Consider a requirement for registration of every purchase and enforcement of the rule of only one ounce per month across all stores in a state. This would be expensive and would place real barriers to using retail stores, ultimately resulting in less tax revenue, more need for enforcement, and perpetuation of the illegal market.

Thus legalization without excessive regulation or taxation is the only way to eliminate the black market. And this approach has the added virtue of maximizing tax revenue from legalized sales, minimizing enforcement costs, and respecting the freedom of those who wish to consume marijuana.