Drugs are certainly an arena where libertarians and leftwingers shake hands on and agree. It's gotta be something to do with the mental damage the drugs are causing them.. Both their ideologies in general and especially on the drug topic.
With the Greens and LP parties joining up recently to fight Bush, I suspect this trend is kicking into high gear. Sometimes I find very little difference between extremist DU types and anarchist libertarians.
I wonder how the use of medical marijuana plays into people who use pot being mentally damaged.
The issue I have with the criminalization of pot smoking is that it A. takes up way too much of the legal system's resources. B. makes criminals of people who have committed no harm to society. C. encourages the corruption of the judicial system. An example of C is laws which allow the seizure of the property used in drug dealings. I am sure most people have stories of the abuse of this provision.
I just don't think pot is a great enough threat to general health, safety and welfare of the public to justify the laws regarding its use.