The logic here is that studies about the effects of a substance on youth has no merits in a discussion about the legality of adult use. Studies about the effects on adults would be more relevant.

The legality of adult use makes these substances more readily available to children, so their health effects on children are relevant.

But it's the illegal one that's easier for children to buy. Drug dealers are already breaking the law, they don't care if the customer is 14.

Can you establish that marijuana is more dangerous than alcohol? If so, I'd be very interested to see your sources and metrics. If not, I'd be very interested to see a justification for why marijuana should remain illegal.

No, I cannot. But so what? Are you saying it is harmless? I don't care if it is merely less dangerous than alcohol.

I did not, and am not, saying that it's harmless. That would be ridiculous.

But if it's less harmful than alcohol, a substance which is legal for consumption and moderated by the government, why should it be illegal? What's your justification for saying marijuana should be banned for reasons of harm, when you can't establish that it's more harmful than more easily available alternatives?

Or are you also arguing that alcohol should be banned, as well?

Edit: To answer your initial question, the logic here is "One harmful substance is legal, thus other substances which are less harmful should probably also be legal".