There are partial transcripts at the links, but I recommend listening to the interviews if you have the time-- there's considerably more in them.
Short version-- Kai had a history of losing a lot of weight and regaining it, so one of her friends thought she had a chance of being the first woman to win on the show. Kai wasn't familiar with the show.
She was also trusting enough to not believe that the contract for the show was really as awful as it sounded. She has since decided to risk being sued for breaking the non-disclosure agreement. The penalties in the contract are very high.
She didn't have an eating disorder until she'd been on the show for a while-- there was a lot of hatred for fat there.
The Biggest Loser is like something out of Norman Spinrad-- the entertainment industry being fraudulent and irresponsible. In particular, not only is treatment of contestants abominable (extreme exercise on little food, no electrolyte replacement, incentives for severe dehydration for weigh ins, medical advice ignored), but the public is lied to about the rate of weight loss-- what's presented as a week on the show is actually a longer span of time.
That last has consequences-- personal trainers discuss the extent to which many people now believe that rate of weight loss is feasible and either expect themselves to achieve it,
Reading about The Biggest Loser made me realize what's wrong with "it's just a matter of calories in vs. calories out"-- that common phrase implies that what happens to the person is of no importance as long as they're losing weight. The cruelty of The Biggest Loser isn't a coincidence.
One of the reactions to The Biggest Loser is "it's just a reality tv show-- people should have some common sense". The truth is, a lot of people don't have much common sense, and big flashy lies hooked to strong motivations will make people tend to have less common sense.
One thing which should be much more common knowledge is that contracts aren't written in stone-- if there are provisions which look unsatisfactory or dangerous to you, you can negotiate about them. This doesn't mean you'll win, but it's better than just letting them slide and hoping they won't be enforced. Also, don't take the word of the folks you're negotiating with about what those provisions might mean.