The lesson here, of course, is that McCain doesn't realize how little people care about his opinion I don't think that any sane person would think that the government was accusing all veterans of being right-wing terrorists. While the military, with its license to kill, would obviously attract people with extreme right-wing beliefs and a thirst for blood (like Timothy McVeigh, who McCain would treat as an isolated incident), nobody in this administration is likely to think that most soldiers are like that. What they are obviously getting at is that soldiers who have been conditioned to see foreigners as the enemy and who are experiencing the psychological side-effects of war are going to be vulnerable to people who would feed them a very "us vs. them" view of the world.
As for Vietnam veterans, many of them have serious problems. There were war crimes that were committed such as the My Lai Massacre, and of course those in the military who tried to stop the crimes were criticized and subjected to death threats. Even those who did not participate in the war crimes and those who did not participate willingly were subjected to the horrors of war and suffer all the problems that come with it such as PTSD. Adding to their suffering is the fact that the government has repeatedly tried to downplay the existence of PTSD and deny that soldiers have been coming back damaged psychologically. For many soldiers the war never ended, and so if they encountered people who convinced them that they were still at war with foreigners then they might be pulled in to such militia groups.
John McCain seems to have hitched his wagon to the idea that everything we know about the past half-century is wrong. Vietnam was a noble cause and would have been a success if the Democrats hadn't stopped it prematurely. Our military never commits war crimes, and never has. We don't torture. The US has never exploited the Middle East to steal its oil for our own selfish desires. Forget that the well-established truth is not on their side, McCain and the entire Republican Party would have you believe that we've been totally wrong all these years. I had really hoped that the Republican failure to dupe people into voting for them would have meant an end to these stupid lies, but obviously McCain has decided to stick around well past his welcome.