Well, doing it as a teenager can trigger schizophrenia, depression, BPD, and a few other mental illnesses. Inhaling smoke is also pretty bad for you. If you find yourself in a bad mood over 50% of the time, seek help. It's a depressant, like alcohol, and that's no good for a developing brain.

A possible reason is because they base their morality on the culture they grew up in, and that culture only promotes the use of alcohol and caffeine (hypocritical, but such is life.) Most people don't bother developing their own opinion on things they don't care about.

Lastly, people don't always agree with the idea of them doing drugs, and morally accept it as a crutch to survive the modern world. Along those lines, they don't want to think their children do drugs, because it implies they failed at protecting the innocence of youth. Also, they have to start explaining moral grey areas, which most people have a hard time thinking about at all.

Lastly, it's a sign of one's child entering the parts of reality a parent can't save them from, where the life of the kid is in the kid's hands. The chance that their kid will fail at life is the nightmare that keeps a lot of parents up at night. This makes parents try to micromanage their kid's life, which rarely results in anything good.