I have a hard time believing US Airways doesn't have a dress code policy. They catered to a lot of business-type people when I worked for Piedmont years ago and were a little more 'uptight' when it came to appearances. Most airlines have a policy that if you are wearing anything offensive or 'inappropriate' they have the option to ask you to change clothes or remove you from the flight. I think the article even mentioned that people were offended by this guy's appearance. That alone would give the gate agents latitude in having the guy either change or removed from the flight.
If I were the captain of that flight, I would definitely have had the guy removed if he didn't put on some decent clothing.