They have 24 hour access to medical care. Sure, every day they drop weight that fast is unhealthy, but every day they spend being obese is unhealthy. Either way their body is strained; might as well get to the healthy zone as quickly as you can, while entertaining and inspiring people and potentially winning money.
I think it's reasonable for people to criticize her weight, being that fitness is the focus of the show. She looks small enough that it's a little concerning. But it's a competition, she won, and she'll probably put a little weight back on now. Whatever. I like dollars.