Why is it acceptable if k-pop stars are dangerously underweight?

Can we just talk about this? I am seriously wondering, is it a cultural thing? Because I rarely see people talking about it while in Europe there is constant debate on the weight of models, who often have a higher weight than k-pop stars. In here I really hope you will not take this as an attack on your favourite artist because it really isn't. When the first plus size model in Korea got attention there was a lot of backlash of people saying she was not a healthy beauty ideal. But neither is SNSD... And I was just wondering, are they naturally like this? Are they constantly on diets to maintain this?

So only Tiffany would be considered healthy. Of course bmi is only an indication, and some people can be healthy with higher or lower BMI's! But sometimes k-pop idols are really extremely underweight, to the point where they are 2 points below the BMI requirement to be admitted to a facility for eating disorders with a diagnosis of Anorexia Nervosa... A BMI of 15.6 is considered healthy for girls up to 11 years of age, from the age of twelve it is considered underweight, and that is exactly the BMI of both Sooyoung and Yoona.

So why is this considered acceptable and normal for k-pop idols? What do you guys think about this? Is it a concern? Or do you think it's okay that the beauty ideal is quite an unhealthy image?