Of the people that wouldn't use drugs there will be some that legality is their only reason for not consuming. But most in the non-consuming group wouldn't consume even if it was legal for a myriad of other reasons.

I prefer the warnings to be helpful for informing adults weighing risks in their decision. Scarier warnings like on cigarette packs are a means of forcing personal or governmental ideals on others through fear. Provide all the known facts and let people make their choice.