It goes far beyond personal behavior. We’re talking about people who waste themselves away contributing nothing to society and becoming a liability and burden to the rest of us in many ways.
We force people to attend school by law so they can become better members of society. There is no other reason to do that. Drug use is at the very least de-educational, ruining your cognitive ability and employability. It makes no sense to have a society that makes people get educated and then permit them to become stupid. We need drug laws to make sure people are minimally functional members of society.
There are countless laws meant to protect people from themselves. We mandate gun training for example before someone can buy a gun. Same with training to drive a car. Your home has to pass the fire code. Whether you’re going to do yourself self-harm, someone else is going to harm you, or an animal or inanimate object is going to harm you is totally irrelevant to the morality of the situation. Society needs to protect people from any avoidable dangers no matter where they originate. Drugs are one of them.
Prohibition only creates more violent crime and no argument will ever convince me that you,the government or any other individual has a right to tell me what i can or can’t put in my own body. You’d think the lesson would have been learned with liquor prohibition. People are going to smoke/drink/etc regardless if it’s legal or not..and the results of criminalization lead to the Mafia, the Mexican Cartels etc.
The argument about “society” is typically what totalitarian states use to justify controlling people.