Question regarding the title of this thread: How does his being a "former active duty Marine" have any bearing at all? Does he get extra 2A rights for that?
Noting he was in the Marines previously or that he as active duty in the Marines previously has nothing to do with the issue. However, the supposed anti-gun "liberal" media that many gun owners think (and often state on this forum) is trying to make gun owners look bad by pointing out transgressions maybe isn't so anti-gun. The anti-gun issues often in the media seem to be sensationalized quite a bit. Gun ownership and the 2A are hot button topics. The more sensational the story, the better for the media.
In this case, the media is sensationalizing the event by noting that the gun carrier was previously in the Marines and/or previously active duty in the Marines. That aspect was noted in many of the versions of the story available online, but why is this mentioned? Simple - the notion of a person being deprived rights when that person served his country to protect them is a great angle to play. How dare the polling officials and cops turn away an American hero? But they weren't turning away a hero. They were turning away some guy with a gun. Having been in the Marines wasn't even part of their consideration.
I realize that the guy tried to cover the bases with both the cops and polling officials by carrying a copy of the state's law in hopes that if there was a problem, then he could allay their concerns once they saw what the law actually was. However, in this case and a few others that I have seen like this, I can't recall any of them being successful that once the laws were shown that the people (usually cops) claiming there is a problem then changed their minds. If anything, providing those folks with a copy of the law only seemed to have deepened their resolve at that time and letting the gun carrier take the issue to court.
If I believed as those polling officials, fire chief/LEO, and officers believed, I would likely dismiss the information provided by the gun carrier as well given that I could not assess the validity or authenticity of the information being provided.