i just don't think its a good idea for heroin to be freely available to anyone who cares to try it. if you are addicted already, and it means you stop hustling to pay for it, great. if you are just curious to try it to see what the hype is about, i don't think you need access to heroin, that isn't reducing harm. by regulating the sale of drugs, you can restrict their accessibility in a logical manner. e.g. people wouldn't be able to buy 10 tabs of acid every day, people wouldn't be able to buy 10 grams of mdma a week, people wouldn't be able to buy highly addictive drugs like heroin, unless they are already addicted. i guess forms of amphetamine which are less neurotoxic than meth would be sold, but i don't think it would be a great idea for people to be able to smoke or inject shard, but that might be idealistic.
if the prices for all these drug services is much lower than the current black market, and there is intelligent restriction of personal recreational amounts, i think it would work better than purely anarchic "do whatever you want to your body".