Because it's enforcing a standard of beauty, if every single woman has the same footwear. Think of it as "options for elegance," if you will. The poster shows that there's a huge variety of formal wear that women can wear and still look sexy; from skirts to gowns to pantsuits. But yet the only "elegant" footwear that the artist could come up with was stiletto heels. So he's replicating a rather dim-witted and narrow standard of beauty in that case. And this particular standard of beauty (read: three-inch spike heels) is impractical, limits mobility, and can cause both short-term and long-term injury to a woman's feet, legs, and back.

There's no denying that heels - particularly stiletto heels - are sexist. And there's a lot of literature out there why, and I'm not going to go Feminism 101 here. But, like other sexist things (porn, stripping, a gajillion other items of women's fashion, etc), they can be reclaimed as legitimate fashion statements. But that's not what's going on here. I can understand if some of those women were wearing stiletto heels, but all of the women (with visible feet)? That's out of character, undermines the individuality that otherwise comes across for each woman, is lazy on the part of the artist, and yes, for those reasons, it's sexist.