It makes perfect sense. I myself find it difficult to accept that it's in character for most of the people in my fandoms to enter into anything like a manner bdsm lifestyle, although more informal power-exchange dynamics pretty much always make sense. What's interesting about the BDSM AU to me is that once you make it the received practice for everybody, you can stop worrying about IF they would do this, and move on to the more fun part, which is HOW.