There is no clear and obvious third party harm. All examples of such are caused by related activities that can be illegal without blanket prohibition. For example, driving while intoxicated is illegal, while drinking alcohol without driving is not. The harm caused to children by their parents' excessive drug use is criminal insofar as it constitutes child abuse
through neglect; drug-specific laws are unneeded. By this logic, alcohol, TV, video games, shopping, gambling, cleaning, sex, reading and writing, and virtually any hobby or occupation should be prohibited as some parents may neglect their children in order to focus on having sex, running a business, or building model railways.