2 reasons: 1) there will always be demand for drugs, thus fulfiling that demand legally will reduce the harm caused by low-quality/tainted drugs, the side-effe

2 reasons: 1) there will always be demand for drugs, thus fulfiling that demand legally will reduce the harm caused by low-quality/tainted drugs, the side-effects of the underground drug trade (violence, weapons smuggling and gangs branching out into other crime) and can be offset by taxation (as long as the total price including tax is still lower than what the black market offers). 2) existing users who are addicted can't just quit cold-turkey, they will need a certain dose of the original drug or an adequate medical substitute for a period of time - that dose can be lowered over time as part of a recovery/rehab program but cutting those users off forcibly will hurt them physically and force them to seek out black-market sources.