I would propose that not every S&M, top & bottom, or even B&D relationship is a D/s relationship. Not everyone who's into the more physical aspects of "BDSM" are interested in the psychological side. Just like not all of us who love a good "mind fuck" are especially interested in pain play. S&M and power exchange aren't inherently inclusive concepts, no matter how often they might be practiced together.
Different strokes for different folks! (pun very much intended)