I believe the article said she dropped by on her day off to pick up some things. Seems to me her supervisor over reacted. Should she have worn the shirt later at the meeting? Perhaps not, but while there on her day off to gather a few items?
Who owns the office? Whether she was there on her own time, on her vacation or after her shift; she broke the Code of Conduct. She was warned, and refused to comply with the rules that everyone else has to live by. Now, she's enjoying the consequences of her actions - and crying about being discriminated against. Sorry, the rules that apply to her, apply to everyone. She was given a choice, and she made her decision. In retrospect, it was probably a foolish one.