I will agree with a differentiation between dopers and occasional users.
Yet, I would still say the overwhelming majority of the latter are liberal.
It is a two pronged effect. An underlying liberal bias is associated with the decision to use the drug, and use of the drug reinforces and strengthens the underlying liberalism.
Interestingly, the dopers may have a higher percentage of what we could consider nominal conservatives than the occasional users.
Truth be told, there aren’t really many occasional users.
And you base this "truth" on......?
Of Americans used pot in the last year, 40% had not used in the last month. (http://www.samhsa.gov/data/NSDUH/2012SummNatFindDetTables/DetTabs/NSDUH-DetTabsSect1peTabs1to46-2012.htm#Tab1.1B)