The point is that he’s not yet been found guilty of desertion, whether we think he should be or not. If convicted, he will likely forfeit all pay and benefits, be busted to E1 and given either a BCD or a DD, but the forfeitures can’t be retroactive.
Still, I have never heard of a person who has been a deserter for years receiving back pay. If that were the case, a person who deserted years ago could show up at a base one day and demand a few hundred thousand in back pay. If Bergdahl did receive back pay, it was a political decision, and not a general rule applied to deserters uniformly.