It is also because 95% of the time you are in first or business class so it is an effort to be classier. Everyone gets these benefits from rampers to accountants so it is a wide range of styles and backgrounds. With the leggings I would be interested to see what exactly they were wearing. If they were truly yoga pants than yeah United was right. However (and I am biased on this because MrsWx sells it) if it was LuLaRoe or similar leggings they can be very nice especially when paired with the right dress or shirt. My money is on the yoga pants though.
My FIL was a ramper for American here in Cedar Rapids for a while (one of his 15 jobs) and he got flight benefits. We were never able to use them though since he used them and gave them away before we could use it. MrsWx was pretty sore about that for a while.
This. CWW#2 worked in marketing for Northwest Airlines back in the day when I first met her. Her sister worked for United. She was uber conscious of dress anytime we flew. The only thing surprising in the story to me was the reaction and lack of understanding and the dad's shorts. Sounds like the parents were idiots. I can't think of any airline folks flying employee, family or buddy passes that would even think of wearing shorts or leggings.