I think the implied counter-argument is that they essentially shouldn't be treated because they brought it upon themselves, so what about:
-Smokers?
-Drug addicts?
-Alcoholics?
-Type 2 diabetes?
Did you know the government spends £300 million each year on heroin for drug addicts, to help them off their addiction, yet only 1 in 10 successfully does so?
Therefore, this debate is wider than just obese people.
But fundamentally our universal healthcare system means because that person pays tax, they can destroy there body as much as they want and be treated for it.