Ever watched Intervention?
Many of those Meth, Heroin and Blow users started with Alcohol and painkillers. Is Mary J a gateway drug? Sure it can be. But so are narcotics and booze. Narcotics moreso than pot and booze now simply because they are much more addictive. Last time I checked, Pot wasn't considered addictive in nature, like Heroin or Cigarettes. Nicotine is one of the most addictive drugs out there - perfectly legal too.
Look at how many people (mainly celebrities) who have died from too much script drugs in their system and/or alcohol. How many deaths can be attributed directly to pot?
Maybe I should have used this in my posts:
There's no evidence it's addictive. The evidence that pot can impair your ability to drive is sketchy at best, and not quantifiable like alcohol. That said, the people I deal with, who are solely pot smokers, and while on probation continue to test positive for it, all have one thing in common...they're fucking dumbasses! The fact that they are not addicted, yet risk jail time KNOWING they'll test positive is cod lock proof. When I hear a pot head try to ask for rehab, I tell them as much too. I actually understand addiction, and as you noted, many addicts come by their addiction legally, and honestly through prescription drugs, but pot smokers are just fucking dumbasses.