Sounds like a variation on the "no true Scotsman" fallacy. My own personal experience with weed smokers is that they often gravitate to other highs, and they no longer bother with weed when they can find something better.
I used to know a pimp that hooked young girls by starting them off on weed, and within a week he had them smoking crack. Weed was just the icebreaker.
Thank you for the advice; If I ever become a pimp I will work this into my overall business model. Sounds like a great idea, /Sarc.
PS: I oppose legalization because I do not want society to condone pot use for kids. I agree that a stigma is a good thing, culturally. The less kids using pot, the better. The more that adults are allowed to make their own decisions, the better too.