When it comes to leggings the fact that no skin is showing is irrelevant. A woman I know described it best. You can still see absolutely everything it is just a different color.
Yoga pants/leggings are not pants. I have had to start having this fight with DD10 this year as she wants to be able to wear stretch pants and yoga pants to school. I am pretty sure the dress codes in our local middle and high schools do not allow them to be worn as pants. And even if they do, I do not find them acceptable unless she is wearing shorts or a skirt or something over them.
After reading the article the girls in question were not the employees. I think one was around 10 years old. Denying them boarding may have been a little over the top but really, why didn't her parents see that she was properly dressed when she left the house? My mother works at a
travel agency and when I was a kid she got a certain number of free airline tickets a year so we flew for free many times and there was a dress code we had to abide by. It doesn't surprise me at all that that is still the case.