He was a paid employee of the United States Armed Forces, and he joined as per his wish.
And he left as per his wish.
No one forced him to join the USAF. But once you join the USAF, you have to abide by the laws and regulations, which are outlined by it.
And what if he only realised the extent of the crimes committed by the US after arriving in Afghanistan? What if this was the driving factor? I don't know that this is what happened, but hypothetically?
Perhaps he also learned that there is no nobility or honour in serving power.