I understand why people might say that and I don't actually know how medics would go about classifying a disease so I'd probably agree with what the 'experts' say about the matter.
But personally, it is not a disease - it's a lifestyle choice and a bad habit. I think there are rare cases of diseases which would cause someone to be obese - but this just proves obesity is the effect of something, not a disease itself.
Though of course metaphorically speaking, obesity certainly is a 'disease to society'.