I have experienced having helped people from various fields complete their written requirements. My clients, just like ProfessorVerb, varied from the academic to the professional. All of them had one thing in common. They all knew what they wanted to say, they just couldn't put it into writing.
I had students coming to me with their outline and almost unreadable notes, begging me to help them put their ideas into an understandable form. Since they did most of the work already and just needed help to put their work into a more academic presentation, I was always more than happy to help them out. Do I consider that helping them cheat? No. Why? Refer to my previous statement.
I also had students come to me asking to do their work from scratch because they had the material, but no idea how to use it. I helped them. Why should their grades suffer just because they are not capable of using the materials they have on hand? Did they submit it for a grade? I pretty much did not care. My only part in this instance was to make sure that they had something usable, for whatever purpose they intended it for.
As for the priests who asked me to write their homilies, the lawyers who asked me to write their case briefs and summations, and nursing and medical students who asked me to complete their case studies, These were people who simply did not have the time to do the work between their classes, regular jobs, and other chores. Who was I to say I could not help them? Of course I did.
Academic writing and ghostwriting are the same in my opinion. There is no fine line dividing them because both of the practices have the same purpose, to help a person get his thoughts down on paper in a manner that can be presented to other people for reading and consideration.