Well, if they are signing up to be on the show and they know what it is, they share some of the responsibility in the effect on their lives. If there's a big prize, it sounds like a great motivation. No doubt many will lose some weight whether they win the final prize or not. That may make them feel better about themselves.
But I'm just sick of "reality TV" in the first place. I used to like some of them, but I'm so sick of it now. Give me a drama. Give me a plot. Give me a compelling story from a writer over a bunch of people gossiping about each other. If it's a game show (and that's what most of these boil down to), do the contests or whatever. I don't need to learn what these people's personalities are and hear them gossip about each other. Show what they're doing to lose weight and have the weigh-in every week and show who gets voted out.
Recently I'm hooked on "Lost". It looks like it's leaning towards some sci-fi/horror film. Aside from the "monster" on the island, this is what I wanted to see from Survivor - a bunch of people stranded on an island trying to survive, not backstab each other.