This is unfortunate because there are many health risks associated with being underweight. First, the general mortality for those who are underweight is higher than for those who are normal weight or overweight. Those who are underweight are more likely to suffer from nutritional deficiencies such as anemia. The underweight are also more likely to succumb to viral illnesses like colds and the flu, and when they get sick, they tend to get sicker and have illnesses that last longer than those who are heavier.
The same holds true for injuries. Injuries are slower to heal in the underweight, and they are more likely to develop infections in wounds. Being underweight is also a risk factor for infertility.
As one ages, being underweight seems to be an even greater disadvantage. Those who are too thin are more likely to develop Alzheimer's disease than those in the normal/overweight range. Osteoporosis is another condition listing being underweight as a risk factor, and if you develop cancer, heart disease, or a kidney problem, having extra body fat increases your risk of pulling through alive and healthy.
My point with this is that it is unfair of the public to criticize, ostracize, and penalize those who are overweight for their body size while actually praising those who are underweight. We should accept all regardless of body size while encouraging healthy eating and movement for all.