It makes perfect sense and I often ask myself the same things.
I think it's a combination of really not knowing proper nutrition, doing "what you've always done," and not having the support you need to change. Money plays a part, too. It's cheaper to eat crappy food, which is an issue for many, especially in the US.
Change is hard. Eating habits/exercise are no different than addiction to substances. What concerns me about the BL is that the people are put in an artificial environment where they have nothing else to do but lose weight and exercise at a level that even I don't approach most of the time. While this may be no different than someone going to a substance abuse program, my concern is that when the person goes back to his/her home, they are still surrounded by people who eat horribly and don't exercise. The #1 thing that addicts have to do is stay away from people and environments where drugs and alcohol are being used. It's a lot harder to stay away from people and places that are eating/serving unhealthy food. It's everywhere.