[joe_the_user]

absolves overweight people of much of their responsibility...

So our object, scientific explanation for obesity is ... lack of personal responsibility!

I suspect Fitocracy is selling their explanation rather than your "explanation" because at least their explanation seems to offer a rational way of changing the situation rather being a mere gesture blaming people (not to say that their system would work for everyone, everyone's different and some people might even find shame and guilt useful for losing weight - still it's unlikely to be an appealing marketing campaign).