There is an odd theory that making illicit drugs easier to use will make them safer, such as needle exchanges and safe injection sites. Other reformers argue that decriminalizing small quantities will avoid court costs and incarceration costs. Moreover, the proponents of the above theories believe the policy known as The War on Drugs has been ineffective and even racist.
There is another theory, one that is grounded in reason, common sense as well as backed by empirical testing, that demand curves are downward sloping. Similar there is an economic truism that posits if governments wish to encourage some activity, then subsidize it. If government wishes to discourage some activity, then tax it.