there seems to be some problem distinguishing between those who would go out for an occasional night out and take a small amount of any drug on their night out (be it an illegal drug or alcohol) and those who are addicted to any drug (again either illegal or alcohol)
Occasional drug use is normally done by people that hold down a steady job, have no problems paying for their weekend habits (and if they are skint for a week they don't go out) and don't turn to crime to fund their drugs (I have known a lot of people that fall into this group, myself included in the past)
then you have the full on addicts (again either to illegal drugs or alcohol) who are those who can not function in everyday sociality, are likely to be out of work due to their habits and will very likely turn to crime to support it.
the first group are doing no harm to anyone (apart from giving money to criminal gangs because that their only choice) the second group are a huge strain on society and are the ones that either need help or need locking up (dependent on person, some would like to get themselves clean, they need help, some don't want to help themselves, they need locking up for the protection of everyone else.
It's also worth keeping in mind the whole reason most drugs are illegal is due to American paranoia causing them to swing their weight about by threatening sanctions on the rest of the world if they didn't follow them.