How do you know the model is underweight? Plenty of models are naturally petite with tiny frames. Also Americans eat too many carbs, carbs are a big part of the obesity crisis. I really don't see your point...I think you're too obsessed about weight
The subject of weight is so intertwined into the culture it takes effort to single out any one aspect. I asked once in the Fashion section what the "Look" is in these days. I got that Healthy is in. Well duh. Yes thin people can get sick just as easily as fat. Extra weight does cause problems statistically but is not the single measure of health. I don't understand particularly about women calling somebody fat is an insult, calling one skinny is also an insult. Personally I think the obsession with weight and "health" is itself unhealthy.