NRSA Dress Code...

All travelers are expected to dress in accordance with the standards of good taste including being 'neat,' 'clean,' and 'well groomed.' The following attire is unacceptable in any class of service.

- t-shirts
- workout attire
- sweat clothing
- bare midriff
- micro/mini skirts
- sheer/see through clothing
- tank tops
- beach clothing
- beach footwear
- cut-off clothing
- clothing with offensive terminology/graphics
- bare feet
- bathing suits
- halter or bra tops
- sleeveless muscle t-shirts
- leggings
- sweatshirts
- flip-flops
- clothing with holes/ragged edges
- provacative/revealing clothing

In addition to the items listed above, the following is unacceptable attire in First Class and Business Class:

- jeans or denim clothes of any color
- athletic or tennis shoes
- hiking/military style boots
- shorts
- baseball caps
- skin-tight or form fitting pants without mid-thigh length top

Customer service judgement will prevail in matters pertaining to dress code.

The above was from back in 2004, but pretty sure the same rules still apply today. I'm guessing those that don't follow the dress code are those who aren't the UA employees, but those traveling on the benefits of a UA employee and likely are unaccompanied.