You should model yourself as a rational agent that makes mistakes sometimes. Suppose it is the case that, if you had heroin on your shelf at home, you would inject it on 0.1% of days. Maybe because you are especially sad/sick/in pain once every few years. If this were the case, you would not want to buy heroin and put it on your shelf at home. 99.9% of the time you will have the willpower to not inject it, but you still expect yourself to be addicted within a decade. The point of bans is to decrease that to 0.0001%, except for those who are actively seeking it.