Yeh I think that this is the case but I also do not think it is highly addictive.
I find that I only feel the pressure to click my joints when I am board, when I am trying all my will power not to or when I have just previosly popped
In the summer I work as an abseil instructor which is tiring work and never boring and during that time I NEVER feel the urge.
The only reason I think that people find it so hard to stop is that it is so easy to do, more than any other addiction. Alcohol, Smoking etc. you have to go out and get cigerettes or a drink whereas for us all it requires is a flick of the wrist, neck, ankle etc. and it is done.
Also for me sometimes I just crack without activly thinking to, like when you start whistling without noticing that you started.
These are just some of my experiences but I hope they are useful