I agree with him the costs are large, and not just monetarily. But loss of freedom. America was created to be a free nation where the individual could make decisions and also the individual would be responsible for his or her own decisions. Aka liberty.
Whether or not marijuana is harmful or helpful should only be an argument to disuade or persuade people to use it, not an argument for outlawing the plant. That is if we are free adults. If on the other hand we are going to be a nation that rules over its subjects for their own good, aka social engineering, then those arguments matter for whether or not the plant should be legal for adults or not.
We never openly had that debate in America.. and infact it was the very socialist leaning people that came in during the depression like Roosevelt who outlawed drugs, and brought us incredible amounts of regulations that the nation hadn't known before.
In the abstract, your point is well stated. But the reality is that few realize that freedom is not free. It comes with the price tag of exercising responsibility to oneself and to others that may have to deal with an exercise of poor judgment. Failure to exercise freedom responsibly almost always results in idiot politicians stepping in and curtailing freedom, and even worse taxing the hell out of the rest of us. Law enforcement costs arise because someone endangers another through irresponsible exercise of freedom.