Obama Is A Pot Head!
I think one reason for is what most voters do with information about candidates' stands. Contrary to the assumptions of all the makers of the "who should you vote for" quizes, people don't have a set of policy preferences and then pick a candidate they think will support those policies. Instead, people look at politicians' issue positions as indicators of their character. They ask what kind of person would have taken that stand?
This kind of character analysis is not unreasonable. Candidates' explicit policy plans never survive first contact with Congress, and there are always unexpected events that the candidate can't have offered a plan for. So we should be paying attention to the kind of judgment candidates exercise, and the core values and dispositions that drive them. To stick with the Obama example, I find his early stumbling on the question of "clean coal" plants much more informative than the promises of X% renewable energy by 20-whatever in the policy paper ghostwritten by his environment advisor.
The problem lies in the mental templates people use to transform a policy stand into an inference about character. The primary template that most Americans have for "the kind of person who would advocate legalizing marijuana" is "irresponsible hippie" -- especially if the person is, as in the case of Obama, already seen as young and liberal. So a voter could easily think (subconsciously or not) "I happen to agree on the merits with legalizing marijuana, and I trust that in my case it's for good, responsible reasons. But the most likely reason that Obama's advocating it is that he's a dirty hippie, and despite our fortuitous coalition on the marijuana issue, I don't want a hippie to be the leader of the free world."