For many years, obesity has been the only medical condition where it is socially acceptable to make fun of the afflicted. On the Biggest Loser, it's fun entertainment to humiliate fat people by making them run around a horse track and mock them when they fail. That's seen as good fun, but society would rightly have a fit if someone suggested a show where cancer patients compete for treatment. That double standard is wrong.
I see it as more of a self acceptance movement. It's trying to say that you should feel good about yourself no matter your size. It's not about disregarding medical advice, or purposefully avoiding improving your health. You can love yourself for being you and not feel like a reject or outsider because you don't fit into an impossible social ideal. No one is encouraging people to get fat, no one is dissuading people from losing weight. People come in all shapes and sizes and that needs to be accepted without humiliation for not falling within a socially approved range.