There's a difference between physical and mental addictions...

Most(if not all) drugs are addictive, after a fashion...even the ones that your doctor may prescribe for you. Cigarettes, for instance have a physical addiction that lasts for 21 or so days after the cessation of smoking. The mental addiction, however lasts much longer, and is much harder to get rid of. Oral fixation, etc...something in your head that makes you feel like you should be holding something (like a cigarette, for example), or that you should be feeling a different way than you do. That's mental addiction. Fine line, maybe, but it's still a line. That's where willpower comes in. Given sufficient willpower anyone can quit using any drug, but only if the physical addiction isn't too severe (such as heroin...hence the comment, "they keep taking it to keep from getting sick"). And anyone can prevent drug use from becoming overwhelming, under the same set of mental/physical addiction properties...hence, the 'casual drug user'. It's a Catch-22 in some respects, and varies greatly dependent on the user, but if you play with fire...

As far as the legalization of any substance goes, it's becoming a frustrating arguement, since most people forget that 2 of the most potentially harmful drugs are legal in the US (alcohol and nicotine), and any other substance, despite the studies, etc. done on them, are automatically villified, although I don't know of any potheads that beat the **** out of their wife/husband and kids, then go run down a half-dozen pedestrians in a blind rage. (I just pictured the party scene from the movie "Reefer Madness", for some reason).
Simply put, I think that if it exists in nature, legalize it. (And don't go throwing that 'heroin comes from poppies' thing at me...i've never seen anyone smoking a poppy seed cigarette) I'm thinking more along the lines of 'pulling my car over and grabbing a handful of ditch weed or a handful of psylocibin-containing mushrooms' kind of existing in nature.

Some people are addicted to money, some to alcohol, others to power, and some to nicotine. Life itself is a series of addictions, some 'permissable' and some not. Either way, I'll keep drinking a beer and smoking a joint when I feel like it, and just watch the static arguement rage on around me...hopefully generating enough willpower to not let any of it control my life.

Ouroboros