Because the dead can't respond or defend themselves. That's why you don't do it.

And it's the framing of the statement that is the problem. They didn't say "I disagreed with Scott" or "I didn't like Scott"; they framed it in a way that made it seem like truth. "the entire arc of Scott Adams is a cautionary tale" makes it seem like he did something wrong and there is some universal truth to be had, when it's really just this person disagreed with Scott's political views. It's persuasion, which ironically I think Scott would have liked.