Heroin users are more likely to OD on a purer shot than something that has been adulterated/cut. So much for ‘unregulated supplies’ being the risk factor.
Actually that proves the point you are arguing against. They OD on the purer product because they are not expecting it, and there is no labeling on the product, so they shoot their usual dose and find out too late that they shouldn’t have done that.
If it was legal, the potency would be printed right on the label, as it is for booze, and people would be less likely to make that mistake.
The reason a heroin user might be more likely to overdose on a purer shot is because they are accustomed to using the drug in adulterated form. If they are not aware that the particular supply of the drug they are preparing to use is more pure than they are accustomed to, they will likely use the same amount of the drug (which is not the same thing as the same dosage); and thereby ingest a much higher dose than they are accustomed to.
Unregulated supplies of widely varying purity ARE the primary risk factor for heroin overdose among users. Some would call that risk the "cost of doing business".