You speak as if kids aren't doing drugs;
As already said they have now and have for a long time. Let's look at other things children also have:

Alcohol, because I am sure their parent keep that under lock and key. I am sure the number of alcohol related injuries, rapes, violence, and deaths are greater than the number cannabis has caused. 

Pharmaceutic Drugs, Opiate, Benzo, Tranquilizers, Amphetamines, etc... Kept under lock and key as well? Once again, I am sure the number of pharmaceutic drugs  related injuries, rapes, violence, and deaths are greater than the number cannabis has caused.

Inhalants, An aerosol based propellant or gas. Kept under lock and key as well? These can be found any and everywhere. Kids die from "huffing".

Why do kids do this? The answer is simple and hard to swallow for some. Bad Parenting. Why is it any industries fault your child is like that? Scapegoats are always easier to blame than yourselves. Teach your children, rather than neglect them.