HOWEVER, why the fuck didn't she just follow the Airlines rules?
Why did no one tell her she had fallen below those rules until she was already in her seat? And why the fuck do they get to set rules on what you can wear (beyond the parameters of existing law)
I think the point is that it's not a public place. It's a business environment albeit she's a paying customer they still have rules - If they, or a restaurant, or a school, or a business have a clothes policy and she was found to be outside of their rules of what's acceptable then she's in the wrong
Airplanes are not business environments, they are transportation devices like busses and trains - paid for by individual customers who are aware they have to share the journey experience with other members of the public. They're already in enough trouble over their attire restrictions for staff. They can shove their policy, they have no right to dictate that sort of thing to customers. And it's not a case of use a rival instead, because there are limited alternatives for the route you may require. Nor would a rival position make the policy ok - just as it wouldn't if they demanded male pilots only, or no black passengers, or women to fly in Burkas.
Schools is not really an applicable analogy - there are no paying customers there, just staff and children (who are often uniformed for safeguarding, amongst other things). Yes, restaurants do the same, but why do we allow this either? I don't get thrown out of tesco for wearing yoga kit, so why off a plane?