Criminalising the sale just turns what could otherwise be taxed and regulated product (tested for mycotoxins, pesticides, solvents, heavy metals, etc; ++tax revenue) into an unregulated (and thus less safe) and untaxed product sold exclusively in illicit markets.

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.