That's just the thing. Psychosis can be a symptom of mental illness or it can be a symptom of smoking cannabis.
I think the question is whether or not smoking cannabis increases your future risk of psychosis. If it does, then it ought to be highly controlled, if not outright banned. If it does not, but can cause psychotic like symptoms during its use, then regulations should be made that it can only be indulged in in appropriate places.
By that logic we should ban peanuts, or at least highly control them.