DELETED is the right answer.
Just going from the title (having had expectations of a link to a study), I have to disagree with von Schwartz. I'm not addicted to SM … I'm also not addicted to various other things. That doesn't necessarily mean they aren't addictive.
Addiction is ultimately a moral determination. Addiction is persistent repetition of a specific behaviour (defined by the activity itself, an outcome of the activity, or a specific artifact of the activity) to the detriment of self. Some people would say "obsessive" or "uncontrollable" rather than "persistent", but I think both of those things link back to the concept of "detriment", which is ultimately a moral concept. Even addicts can control their detrimental behaviours to a degree. There would be no relapses if there were no stops.
So what is the detriment of persistent SM activity?
Loss of contact with "the real world"? This could be a detrimental outcome. It doesn't have to be, but very few outlier behaviours are immediately and totally detrimental. Most exist on a spectrum.
What about "loss of mental acuity"? I would agree with this one, as well. And it leads to the "why" bit. SM is a fundamentally passive activity for most. But it creates a large stimulus in terms of novelty (one's first experience with material) and in sense of belonging (the broad group of SM participants). Even when true novelty is gone, you still get the repetitive behaviour, much like people watching reruns of shows they can recite by rote. And that circles back to the part I find detrimental – replacing real novel mental activity with an assuaging, but mentally empty activity.
Now, please excuse me while I cut this post short. The Simpsons is coming on …