Do most American women actually look like that as a size 12, though? That woman looks like she's pretty fit and probably has a fairly healthy lifestyle. Let's not forget that a size 12 on a woman who's 5'10 is going to look different than a size 12 on a woman who's 5'4. None of this necessitates that a size 12 is healthy for your height, weight, or build -- especially considering the average height for a woman in the US is between 5'4-5'5 and we have a nation presently suffering from a large population of overweight/obese people.
I'm not saying that we couldn't do with more healthy representations of body types of all sizes, but those pictures are misleading in that I have my doubts that the average woman looks like those examples, either.
More on topic: The VS model just looks like she got shot from a really poor angle. She really doesn't look that much thinner than the other two to me, especially in the other four pictures. So long as she's not doing anything unhealthy to maintain that weight, uh, who cares? Thin women are as natural as curvaceous women. Slamming one does not make the other look better. You can have preferences and tastes, but this whole "real women are X or Y" thing really grates on me.
Besides, as I think it was already mentioned here, it's VS's business who they hire and who they fire. If they want a certain look for a job that is almost exclusively based on looks, then they have a right to demand it.