I think the difference in calling US 'US Air' instead of 'US Airways' is that 'US Air' is not really an abbreviation, it is the name of the airline 15 years ago, and thus it just seems like you are calling it the wrong name.

So, calling AA 'American' is an abbreviation for 'American Airlines', but calling it 'American Air' is just incorrect.

While it bothers me to hear 'US Air' I certainly admit it's not that big a deal.