This is a descriptor that the press uses like no other. You rarely hear "former soldier" or "former sailor" used in front of someone's name. There are two times the press uses it. The first is when describing some lunatic that has committed a heinous act like Lee Harvey Oswald, John Whitman, and Steve Ng. The other is when someone is criticizing the Bush administration. I guess the press feels like Marine vets are all stark raving lunatics as well as foreign policy experts.
In Murtha's case, the description has slid from the latter to the former, IMO.
Don't forget Ramsey Clark, former Marine 1945-46.