I do not agree with banning skinny models. I think the solution to oppressive expression isn't banning things (which is more oppression), but instead, more expression. So, in this context, the answer to skinny models being used to perpetuate unrealistic beauty standards is not keeping the models from working, it's increasing the number of non-pin-thin models who work. If you increase the diversity of body and beauty ideals, it won't be unduly influential that a couple of these models remain skinny. If they want to create a useful ban that isn't based on body shape, may I suggest an upper age limit of 18, or even 20? At some point you'd probably organically reduce the number of skinny girls if you kept the 14-year-olds away from the backstage old-man-and-cocaine parties.
However, I disagree with the above comments, to the extent that they state that banning thin people from modeling is as damaging as scolding fat people. It's not. That's because of context. There's a whole industry and culture that supports thinness as an ideal, and that will exist whether or not people as thin as models are allowed to work. On the other hand, there's a widespread bias in our culture and many others against fat people that affects their ability to get adequate health care, buy clothes that fit, earn as much money as thin people, and exist in the world without constant harassment that most onlookers think is acceptable and "for their own good." I don't think anybody should be banned from working based on their body, but generally out in the culture, that actually happens to fat people and it doesn't to thin people. So, they aren't the same.
Banning women that are legitimately naturally skinny is just as bad as scolding a woman that is naturally curvy. If people really wanted to promote healthy body image they would have multiple sizes and body shapes on the runways. It would represent the vast customer base and maybe make the companies more money.
I always cry foul when the answer to discrimination is more discrimination. How bout include models of all shapes and sizes instead of banning the skinny ones? Some folks are skinny and can't help it no matter what they eat, the answer shouldn't be to make them feel like their physique is a problem. Also Fuck a bee and wasps can eat a fat one too!