This time tweeting United Airlines is discriminating against women wearing retro 80s leg warmers? Discrimination is understandable considering fashion sense but not because they are women.
Employees get so many passes a year they can authorize family members or friends to use those passes, they are sometimes free or unbelievably cheap. I've travelled from the US to UK first class for $200 on a friend's pass but everybody is warned that there is a dress code also a behaviour code. Those girls knew that they were not incompliance with the dress code when they tried to board the plane. It was some woman who witnessed the girls being refused boarding who immediately got onto Twitter without knowing the full story.
The airlines don't have to offer these benefits to employees, it's a fantastic perk for working for the airlines and a small price to pay is that anyone using these passes adheres to the dress code.