Yes, people don't realize that this "solution" doesn't work for any drug just because it makes sense for marijuana. For the people who disagree: In the limiting case thought experiment, imagine a drug which provides a complete DMT + MDMA experience with a single molecule, but has a 50% chance of killing you. Should this drug be easily available in a store? Run a simulation, let me know what the half life of your population ends up being...
Heroin isn't too different, really. It's very addictive, has a low LD50, and when it's easy to get, people effectively get a free ticket to a brothel on an accelerating rocket-fuel train with rarely stops. If a drug can have that much power over the body, people might think we could create microdoses, and sell those, and then it'd be safe. But then, the market for heroin/tar/etc. would still be there, and would continue underground.
It's not as simple as "selling it at CVS w/o prescription will lower overdoses and dissolve the underground H market"