Here the process goes like this... You buy marijuana. Your drug dealer sells you marijuana with some heroin mixed. You smoke it without knowing that the heroin is there, because you're just stupid child and believe in everything they tell you. Then drug dealer increases doses of heroin in you marijuana and you become more and more addicted. It's as easy as that.
Upisoft,
If its "organized" crime, not just petty street supplying then yes, what you mentioned is just one of the methods to facilitate drug abuse and a drug demand used by the drug barons. Thats conventional "gangster" network methodology.
BUT in standard street supply, the heroin is not mixed in marijuana at all, because frankly it sells for too much more, as a luxury. Even with marijuana/weed/ecstacy users heroin is LAST resort as considerd "hardcore", b/cus its highly addictive, expensive and lethal.
Drug "dealers" are only normal people off the street, NOT necessarily some thug dispositioned dudes, and most of them do it as part of survival, society and culture...
I.E. Dad did it, brother does it, sister does it, mother does it, friends ALL do it at different strengths and levels; kid grows older, is VERY desensitised and adapted to taking/distributing illegal drugs by now and carries on the trend, because theres a DEMAND for it.
Its definitely two way, BUT when the parents have only taught them this, what do you expect. One of many scenarios but I've seen this scenario play out to begin a full area into a 'drug network'.
How it works is like women/sex works for many; you do one thing and you move on to what you've never tried but heard pleasantly about

Never heard of the "buzz" or "high" these people will brag about?
Well, thats one of the reasons they'll attempt consuming drugs and then moving on to one with a bigger "buzz".