While the debate about legalisation of cannabis is endlessly fascinating, it obscures the vital question of how to design a system of legal availability.
Prohibition produces some very bad results. It deprives millions of people of the liberty to pursue what would be, for them, a harmless pleasure. It creates an illicit market that delivers tens of billions of dollars a year to criminals. It leads to large numbers of arrests (mostly for possession for personal use) and a smaller but substantial number of incarcerations (mostly for growing or dealing). The black market fuels corruption and violence worldwide.
On the other hand, prohibition maintains high prices, discouraging heavy use and use by minors. The problem is how to shed the harms of prohibition while minimising the harms of legalisation. The desirable outcomes are cannabis available to adults who want to use it in moderation and abolition of the illicit trade but without significant increases in habitual heavy use or in more-than-occasional use by minors. Alas, current legalisation efforts in the US, replacing prohibition with commercial production and sale after the fashion of alcohol, have little prospect of getting us there.