Technically speaking it is not addictive. There are two components to addiction, habituation (habit strength) and dependence (physical withdrawal).
Cannabis meets the criteria for habituation, but does not cause physical withdrawal.
Are some heavy cannabis users habituated? Most certainly, but if they are "addicted" then it is in their psychological makeup and not a direct result of an inability to tolerate the (very mild) withdrawal symptoms of a cessation of cannabis use.
This is my opinion of course, as I am not a doctor. However I did study developmental psychology in college, so it is at least an educated opinion.