It's a little more complicated. According to http://www.burningman.com/press/
Burning Man is a private event, and professional photographers and ALL moving film and video camera users are required to sign use agreements that will be emailed upon approval of a project. Our review and written permission is required to use any image from the event commercially or publicly; no public use of images is allowed without this written consent.
It is possible that the photographer already has permission from BM, but needs it from the subjects. It is possible that this is a case where a picture not originally intended to be commercial now has some commercial value. Other possibilities exist, no doubt. I'd be slow to judge, here.