Well there are a few things going on here, and I don't even think the trans deal is high on the list. First, like Bristol Palin, it's all about getting someone else who can draw ratings to show up, be it Sara or Cher. Secondly, fat people have a high entertainment value, which is why you tend to get one per season of DWTS. They sweat, they fall down, they tend to be so graceless that they make other people feel good about themselves. It's one of the last forms of PC acceptable humor directed against other people. See also: American's Biggest Loser.
What's really through the looking glass is that the only stars on DWTS anymore are the dancers themselves. It's all about them trying to overcome the celebrity handicap they've been given.
