As noted in Gault's article, this is NOT the arbitrator's full decision, which has not been released yet. Rather this is just the Operative Award. A full decision that lays out the evidence presented, the parties arguments, and the arbitrator's thought process will be released in the future. Note the Operative Award states that "(the reasons will follow in due course)."
The article also explains that, due to the timing and high profile nature of the case, that Tygert thought it was better to have the arbitrator make the determination regarding accidental consumption instead of USADA so that it would be viewed by WADA and the AIU as more valid. Now, if USADA somehow tanked the case, that is highly problematic. But if it simply presented all the evidence it gathered and let the arbitrator decide, then that seems appropriate. The accidental consumption is essentially an affirmative defense. There is no dispute (as far as we know) that the trenbolone was in his system. Thus, USADA proved its case. But instead of USADA internally deciding to exonerate Knighton based on his affirmative defense - it was in the meat - it allowed an independent arbitrator to make that decision.
This is different than a criminal prosecution, where cases are NOT supposed to brought unless the prosecutor believes reasonably believes that they will be able to achieve a conviction beyond a reasonable doubt. Thus, a prosecutor should never be happy with a not guilty verdict, because the prosecutor should not have brought the case unless they felt strongly that the accused was going to be convicted.
In this case, it seems like USADA believed Knighton's excuse - the contaminated meat - but had the arbitrator decide to avoid the appearance of favoritism. If this was in the context of criminal law, the charges simply would not have been brought (i.e., someone who kills another but where the killing was clearly in self-defense).