The way I see it, it is very simple:
If the photograph is there just for the purpose of being a photograph, or to sell itself as a photographic object, it is fine art.
If the photograph is used as imagery to sell or promote something else beside the photograph itself, it is not fine art, but commercial.
If it is neither, then it is neither. e.g. family album pix.
This being said, photos that were once fine art can be later used for commercial purposes (and vice versa in some cases), and stylistic crossover is not only possible, but extremely prevalent. Therefore, it is use and purpose of the images, not the style and/or content that makes the determination.