We take great pains to educate people on why they shouldn't drive drunk. We give fairly detailed medical descriptions of what it does to you. But when it turns to pot, there are people in charge of policing it who still insist that it is more addictive than crack and more damaging than heroine. Really, I just heard this on the radio a couple of days ago from someone in the DEA.
Once people see one friend take a few drinks and stumble around, and another one smoke a joint and seem mostly unaffected (except for the giggling); once they see their roommate light up twice a year to celebrate the end of finals, but not crave it the rest of the year; once they talk to aunts and uncles, or even parents, who smoked regularly through the 60s then gave it up when they left college and started their careers; they start to question what they've been told.
The politicians have been crying wolf over marijuanna for about 80 years.