The reason marijuana is a "gateway" drug is because it is illegal. The same underground networks that deal marijuana deal the harder drugs as well.
With marijuana legal people would have no reason to come into contact with the really nasty guys...
With all drugs legal (as called for in the article) the heroin and the cocaine and the methamphetamine will be on the store shelf right next to the marijuana.
But you'd have us believe that the legal marijuana users would never reach over to buy the legal cocaine -- there'd be no "gateway" effect if all drugs were legal.