I think unwilling is probably a better word. An ex-girlfriend of mine was a good student in college and then started skipping class to smoke pot and failing her classes. Several times she said she needed to quit but couldn't. Sounds like addictive behavior. However, you're probably right in that those same people can become addicted to sex or porn or whatever.
For people in that mental state, pot is likely the least of the evils they can resort to. I've never heard of a person who felt compelled against their will to smoke pot. This hits home with me at the moment because I am trying to quit cigarettes and it is unbelievable how difficult it is. I don't want to smoke any more cigarettes but at times I simply can't control my own body as I reach for one and light up, which is very frustrating and frightening.