1. Putting people in prison for using it causes more harm than the actual usage of it does.
2. The threat of state force being used to strip people of their freedoms is not an effective deterrent. The war on drugs is an objective failure.
3. The war on drugs was nonsensical to begin with. You are legally free to drink bleach. You can legally drink yourself to death on alcohol. You can legally consume enough caffeine to give yourself a heart attack and die on the spot. You can legally smoke 100 packs of highly addictive and harmful cigarettes a day. But you can't legally consume cannabis, which remains a schedule 1 drug?
Legalisation is of utmost importance not because cannabis is good and we ought to encourage consumption, but because the alternative (criminalisation) is irrational, morally reprehensible, and a costly waste of taxpayer resources.
There is no good that comes from criminalisation, only more harm, more waste, and more people being stripped of basic human rights for no justifiable reason.
Again, it's not about wanting to encourage the sale of it, it's that the alternative (criminalisation) causes more harm than good.
To your point, I'm all for having a surgeon general's warning on cannabis products, restricting the sale to age 25+ (when the prefrontal cortex is much closer to finishing development), and other measures that may reduce harm, rather than increasing it.
Your framing of the situation is just remarkably wrong considering the entire legal history of marijuana in the US.