Reasoning: most diseases have a direct "cause and effect" - if you can't eat healthy because you weren't forced to eat anything other than chicken nuggets as a child, that may well be a disease. (My GF suffers from this; she says we will serve mini hamburgers and chicken fingers at our wedding...) But obesity in general, to me, is not a disease.
However, if labeling it as a disease can help people in any way to get on the path to a healthier lifestyle, then it doesn't bother me. Calling it a disease should not be a crutch, but hopefully a way to address the problem. I believe insurance companies will have to cover operations that can help people lower their weight.
I believe in Canada our health plan covers that, kind of bugs me though because it comes from my taxes rather than paid insurance.