Strictly, such a thing is not an addiction, but a habit. Habits can be almost as harmful as addictions, but the treatment for a habit is quite different than for a physical addiction. Physical addiction, once broken, is gone forever. Habit-based cravings don't go away anywhere near as easily, which is why AA encourages sufferers to work on the premise that if you are an alcoholic, you are an alcoholic for life.
I suspect that most people who say they are addicted to marijuana are really talking about this "harmful habit" version of addiction, rather than a purely physical addiction.
You can develop a harmful habit about anything - Alcohol, marijuana, ecstasy, masturbation, brushing your hair, forum posting, computer games, etc.
It's when you prioritise a trivial part of your life over an important part of your life that a habit is harmful. If you skip work and lose pay or get fired because you're too busy refuting woos on JREF, then it's definitely a harmful habit.