On the "It is unhealthy to lose weight that fast" line of thought:
I am 5'10", 210. I need to lose about 15-30 pounds. It would be unhealthy for me to lose that over a month or two, three is pushing the limit on the healthy weight loss thing.
The people on this show are morbidly obese. They can lose probably 2x-3x as much as I can in a shorter amount of time and still be healthy. They have a lot more fat to burn, they have a lot more calories to cut, and they have a lot more moving to do to catch up to where I am not even exercising. So, yes, it is unhealthy to lose weight fast. But they're really not losing weight that fast, especially when you look at it on a percentage basis.
Source: picked up from years of living with nurses, dietitians, and fitness professionals.