Desertion in the face of the enemy is different from some guy in the motor pool in Louisiana going AWOL to shack up with his girlfriend. When I was a military officer at a stateside base, we had the occasional guy stop coming to work; after 30 days, he was technically a deserter rather than just AWOL. If they were apprehended, punishment ranged from reduction in rank, or a fine, to discharge under less than honorable circumstances. We never got around to shooting any of them. Hell, we didn’t even waste time looking for them unless we suspected foul play.
In a way I don’t blame him, Nam was the first war or if you prefer police action that was run by politicians. He did turn his life around, he went on to become a grower and retailer of some decent pot.