Many do. However, if you read State Constiutions, You will be hard pressed to find one that gives prohibitory power to the State.
Or is it because, concerning pot, libertarians don't see any conflict between CA laws and the constitution?
Sure, there is no conflict between CA laws and the Constitution since the Constitution does not grant the federal government the power to ban anything, let alone "drugs". FDR-New Deal "interpretations" of the Commerce Clause do not pass as constitutional authority, especially since the CC gives congress the power to regulate commerce between the States, not ban commerce or private possession of plants or objects.