Yeah, it's just not in good taste to let it be known. But sometimes passengers around you can guess and then to be dressed the wrong way just annoys passengers.
http://www.airliners.net/aviation-fo...ode+#ID3361202
Ironically, seems that one of UA's major competitors basically ditched its non-rev dress code for the opposite reason... because the non-revs stuck out like a sore thumb.