The real question is; why is it any of your business what others do so long as they don't violate your rights?
Free people do not ask "WHY should things be legal?" They ask "Why would we want to make things illegal?"
You have it PRECISELY backwards.
Right next to your 9th amendment "right" to do drugs is my 9th amendment "right" to raise my children in a drug free environment.
Oh, I don't have rights? Only the dopers have rights?