Doing it in inappropriate places isn't a good enough definition of addiction to masturbation.
Doing it despite harm to yourself or others is more accurate.
AKA
People who cancel important events to jerk off
People who neglect their social life in general because of it
People who do it instead of their work (of they see working at home), or eating, or any other very important task.
People you mentioned who "can't hold it in" and do it in inappropriate places.
However some people have kinks for inappropriate masturbation and specifically that isn't addiction. Other people weigh the risks and have a different judgement system. Some people are just much more willing to break the law. It's not good or justified, but not an addiction. If someone stole a golf kart for a joy ride would they have an addiction to doing that?