I stopped posting on PolyPercs when I found my way here, because this forum seemed more active. (Thanks, Kevin, since a link you posted there steered me here.)
As for Ask Amy, Dear Abby, and so forth... Advice columnists are first and foremost human beings, and they don't always have a bleeping clue about anything. They're also prone to being judgmental and sometimes harsh. I remember going all Hulk-ragey at one point when Dear Abby posted a letter from an obese 20-something young woman asking how to get her mother off her back when she wears two-piece swimsuits; instead of giving helpful, maybe even compassionate, or at the very least on-topic advice, Abby said, "Have you talked to your doctor about your weight?"
Like physical appearance, relationship structure is one of those things that people feel entirely comfortable judging each other about and making those judgments loudly known. Why? Who the hell knows?
When someone writes to an advice columnist, they're opening themselves up to the judgment of the columnist and probably millions of readers. An advice columnist usually isn't an unbiased professional like, say, a therapist is supposed to be. So their own prejudices, biases, and dislikes are going to shine through in their replies.
Polyamory isn't widely understood, even, in some cases, by those who claim to practice it. (Witness how many people post things like "is it really poly if my partner and I aren't sharing a partner".) So it's understandable, though unfortunate, that an advice columnist might not have had any experience with or education about polyamory, and therefore default to either the "it's just a fancy name for cheating" idea, or the "relationships are supposed to only be two people, and nothing else will ever work" idea.