"Illegal" being the operative word. Seems to me that if an individual is willing to break the law to get one drug, they'd be willing to break the law to get another.
Who's more likely to "gateway" to robbing liquor stores: a teen who works for a living or a teen who shoplifts? Now, you can certainly make the case that not all shoplifters go on to rob liquor stores. Or even that some shoplifters are actually well-to-do.
But that, of course, misses the point. The point being who's more likely.
Keeping teens away from illegal soft drugs (marijuana, Ecstacy, nitrous, GHB, etc.) helps to keep them away from illegal hard drugs.
"Keeping teens away from illegal soft drugs (marijuana, Ecstacy, nitrous, GHB, etc.) helps to keep them away from illegal hard drugs."
Of course, there's an obvious corollary of that argument that comes to mind: Legalizing common soft drugs could help keep teens (and others) away from the hard drugs.