It is not addictive. They can quit any time they want. They just don't want, amiright? Or maybe they will quit next Monday, right now is just not a good time.
I have a friend who lives with his college mates. They are not in college now but rent a house together (like 4 or 5 people). Says he is the only one of them who does not smoke pot every day.
Sounds like behavior. Or, if we're going to pretend they haven't got any agency of their own, psychological addiction. Something far from physical addiction.