Notions of consent are different. You do what the doctor tells you, and its an enormous ethical violation to recommend drugs without revealing the way you personally gain from it, or to encourage people to recommend drugs that are not medically necessary.
With legalized drugs there isn't this trusted arbiter telling you what to do, so you can consent easier when you buy and take them.
I agree that it's fraught and there are tons of ethical parallels that need to be worked out but that's one reason.
It's sort of amazing that the reason we have these licensed people who are supposed to be "trusted" to give good instructions is because of the system of prohibition on the sale of drugs by the unlicensed.
The argument is, of course, that by restricting the distribution to licensed professionals, the abuse of dangerous drugs would be curbed because the trade would be restricted to to those trained in their safe use.
We have the worst of both worlds. This is all a result of prohibition.
With legalized drugs there isn't this trusted arbiter telling you what to do, so you can consent easier when you buy and take them.
I agree that it's fraught and there are tons of ethical parallels that need to be worked out but that's one reason.