One bit of information revealed in the original story is that the thin fabric pants were selling for $98. On the other hand, yoga pants have become a strong substitute for jeans as comfortable, casual pants in settings that don't require mats or yogis.
Putting it another way, they're on target for people who think their butts are shapely, and want everyone to know it!
But this article in Slate by Amanda Marcotte seems to either miss something entirely, or she was ironically dense:
Why do women find the prospect of wearing see-through yoga pants to be troubling? It's not because of V.P.L. exposure. No, in some settings, a woman showing evidence of panty lines might provide a sign that she is conventional, if not virtuous!
What a woman is wearing under her yoga pants opens her to be judged on different levels by others. (The author herself specifically took women who wear thongs to task.) But, given this fact being open to others' judgments, it's one of those situations in which, whatever choice she made, it's going to be the wrong one to someone else. Some people might feel free to find fault with someone else's choice in a multitude of occasions, not to mention whether she's wearing a thong, a bikini, regular step-ins, or even granny panties! Or going commando, heaven forbid!
Simply put: the women who wear thongs under their yoga pants do so because they're pleased with their asses, and want them displayed in all their glory without the tell-tale V.P.L. shifting the focus from the butt to the thong. If the pants are sheer, the color and pattern of the underwear is obvious to others. Do you want everyone to know that you're wearing pink polka-dot panties? Not usually.
There's another matter: clothing that might be too sheer is going to be typically sheer both in front and in back. There might be some unintentional disclosure that could further lead to a negative evaluation that is definitely morally tinged! That's why some women wear thongs!
calliphygian -- kl-pj-n) also cal·li·py·gous (-pgs)
Having beautifully proportioned buttocks