My understanding is that more research is pointing to obesity as, in some sense, a precursor/reaction to the onset of type 2 diabetes rather than type 1.
Once you get to quite obese you're dealing with physiological factors that make losing weight medically difficult from behavioral changes alone. It also makes the chances of "yo-yoing" the weight higher as well. At that point the treatment for obesity overlaps with the treatment for type 2 diabetes.
There are varying degrees of control over outcomes. The judgment comes from the correlation between an unobserved variable (effort at controlling or preventing obesity) with the observed variable (actual obesity).
Once you get to quite obese you're dealing with physiological factors that make losing weight medically difficult from behavioral changes alone. It also makes the chances of "yo-yoing" the weight higher as well. At that point the treatment for obesity overlaps with the treatment for type 2 diabetes.