Of course, at the time he was in Vietnam he was not a traitor. He was merely a less than adequate sailor and commander.
That said, the awarding of combat decorations has built-in safeguards spelled out by regulation. It seems in Kerry's case these safeguards either broke down or were circumvented by Kerry himself.
Two of his purple hearts and both his bronze and silver stars appear to have been "gamed" by Kerry, who had a working plan to get himself heroically decorated in a short time. False Situation Reports by Kerry himself seem to be involved in all of these as well as going outside of his chain-of-command in the case of his first purple heart.
The unit itself seemed rather lax in the factual vetting and eye witnesses required in the verification required for combat awards. It seems his superiors relied wholly on Kerry's self-serving reports - which is not the normal means for awarding decorations. It would appear to me much of the normal oversight was lacking at his unit level, which permitted Kerry to "cheat" his way to his citations.
Kerry did it very quickly as he was only in each unit for short periods of time so he was never caught at his lies and evasions. Apparently, he was clever enough and his short time at each assignment no doubt helped him to hit-and-run with these lies and evasions and nobody was the wiser. They were on to more important things (to them), such as fighting the war. Being a liar and a cheat is so much easier when nobody is paying attention.