It might be a method for a soldier to get a discharge. Unlike officers who can resign at any time they are stuck to complete their service. In days gone past stating you was a homosexual enabled you to get out early.
There are always, and always will be, soldiers that do drugs, I doubt its as big a problem as the media would have us think, although, as a thought, they are in the cradle of the Poppy trade,
I know 3 ex squaddies who deliberately got caught smoking cannabis so they could get out of the army before the time they signed up for.Its a well known ploy to get discharged.