I don't think the laws are the way they are because people don't have common sense. Think of all the lawyers, drug testing lab techs, court clerks, judges, and probation officers that would lose their jobs if the war on drugs was lifted.
If you ever watched police shows like "cops" they seem to spend 2/3 of their time arresting people for drugs. It's hard solve home burglaries, it's easy to bust people with petty amounts of drugs in their cars.