This article at Mother Jones seems to indicate that even if Bergdahl walked away voluntarily, he was AWOL and not a deserter. Unless it is proved that the soldier has no intention of returning, the soldier isn’t considered to be a deserter until 30 days of being AWOL.

motherjones.com

I think it is a difference worth keeping in mind. AWOL can be accidental or sloppy or a temporary brain freeze or whatever. Desertion is much more deliberate and nefarious.