On a lot of those shows, the overweight contestant is closely monitored by doctors and medics during the entire time. It is extreme and I think it serves twp purposes:
1. It pushes them past where they never thought they could go to show them how tough they are and how much tougher they will need to be.
2. It gets a quick scale reduction result, which is likely to reinforce motivation.
I agree that it look INSANE.
Complication makes it easy to explain failure.