But the OP isn't specifically discussing the parents of soldiers who died in combat, or their treatment here.
Rather, he refers more generally to "people [we] disagree with".
Further, he isn't limiting use of epithets to "scum," "unamerican," "pink-commie" specifically. He was just using them as examples, which is evidenced by his following that list with the phrase "you get the idea".
Essentially, what he's saying, is "don't be a douchebag to folks whose ideas you might disagree with, because they might be really stand-up people in many other respects".
I think he makes a great point.