I was always under the impression that the reason they lump CFS/Fibromyalgia and ME into the same categories is because they are all aspects of the same overall disease/condition.
During the course of me having CFS... Its been extremely severe, to the point that ME was the diagnosis, and then at one point... I had the tender points which is when Fibro was my diagnosis, then it was just CFS. Now I just have suspected adrenal issues.
IMO, I think the reason why they lump them together is because they are all "alleviated" or the symptoms are at least minimized by similar protocols for each condition.