I think it's called brainwashing. Since the 1930s (correct me if I'm wrong) the US government has been telling everybody that weed was forged by Lucifer himself (thanks to Groudskeeper Willy for that line ). I guess even if people initially didn't believe it, after almost 100 years of repetition, most of the population now believes it. I say most, however the movement to legalize/decriminalize marijuana is slowly gaining momentum, even among the more conservative types.

I'm positive that eventually, maybe not even in our lifetime, but at some point, the government will give in. It's just a matter of time.