I once thought that way, until I tried pot for the first time. ThenI realized we were lied to and felt betrayed by the system. I do believe kids should not do drugs as their bodies are not fully developed and may stunt their growth into adult hood. But we all need to make our own decisions on the matter and not to force it on others. It sounds like he still believes in the propaganda he was fed as a child. That constant brainwashing takes years to wear off, as mine did by my late twenties.
It all comes to choice. No one is forcing you to do them.