I think that most of the time it's just a byproduct of people parroting a sentiment without being able to fully articulate the reason for it. Reality is more complex than most people have time for. Many of the replies to that comment were just as obtuse as the one(s) getting all the criticism. For example, no, it doesn't make a person ipso facto immoral for owning slaves in a time and place where this was normal. There is certainly a good deal of hypocrisy between the rhetoric of the Declaration of Independence and the reality of owning other people, but the analysis shouldn't stop there, and people shouldn't be so quick to apply today's standards to people who lived in very different circumstances.

The commenter did a poor job of saying, essentially, that even though they may have ultimately been part of the problem, there were slave owners who were not the caricature of evil that they are frequently portrayed to be today by people (all of us) who find the concept of slavery abhorrent on its face. An example of this was on the front page just in the last few days, although the focus was rightly on the slave, who went on to rescue his family and deliver a Confederate ship to the North.

I'm not trying to rain on the parade here, except to point out that American kids are told from very early on that the founders were extra special, and that people here grow up in an environment where that idea is taken for granted. Of course it sounds ridiculous to anyone who grew up where it isn't so. If you really want to get to the root of just about every SAS phenomenon, I think you'll find it in our cultural aversion to nuance. But I see that in other places too. We're just afflicted more severely, it seems.