This moral position can have different origins. I will consider all of them. Note that I will describe the extremes of continua. This combination seems to be a logical necessity. If something is even potentially, positive, it should not be prohibited.
Society has only to regulate it, optimizing positive effects and minimizing negative effects.
But it is equally clear that at least academically those that take that stance do not have to be opposed per se to prohibitionist systems. As logical as the former combination seemed to be, so illogical this one appears. Apart from underestimating the capabilities of fellow people to handle 'dope', this position is often taken on a basis of fear. They feel they need the external controls the prohibition is somehow providing them. This attitude is however much more common in a a vaguer way.