Just because something is legal, doesn’t mean everyone will purchase it legally. For instance, people still download pirated copies of music and movies rather than pay for them. And gun buyers may prefer to eschew the mandatory background checks and waiting periods associated with legal firearm purchases.
The same is true for marijuana. In the year and a half after California legalized recreational marijuana sales, the state has seized over $30 million in illegal weed, much to the chagrin of dispensaries that are following the 300-page rulebook.