I suspect that US Air was getting something for every sub ordered.

I ordered some subs a year or so ago to burn a couple of thousand left over US Air miles and inadvertently ordered more than my miles would cover. I got all the subs. And, now, US Air is showing that I still have a positive balance in my acct. Kinda weird.

Back on topic: one big reason that miles expire is that they must be carried on the airline books as a contingent liability which can hurt the ability to report profits. Once they expire, reported profits can go up.