Ethics as a core facet of polyamory date back not only to its inception with Morning Glory Zell-Ravenheart, and as covered by Franklin Veaux, but also Hardy & Easton, authors of The Ethical Slut, considered by many to be the Bible of polyamory. There is no escaping the historical significance of this fact and therefore it cannot simply be hand waved as irrelevant.
Given that people have been practicing polyamory for hundreds of years before Morning Glory Zell-Ravenheart coined the term and Franklin Veaux decided to write a lot about it, they are not the people who define it. Franklin and Dossie both would balk at the idea that they are defining the term. They write about what they think the best way to practice it is.
(and that's not even getting into the colonialist view that Morning Glory somehow was at the "inception" of polyamory)
Monogamous relationships don't include an ethical component as part of its definition. It is simply about limiting the number of partners. Nothing else, therefore the comparison isn't coherent.
That's a distinction you've chosen to make. There are a lot more books out there about how to have ethical monogamous relationships than how to have ethical polyamorous relationships. What makes the books you cite a defining authority over the relationship structure while these other books aren't?
To proclaim ethics is a useless qualifier in one type of relationship because it's your opinion that it should apply to all relationships is only logically coherent to your particular viewpoint. In order to have an objective definition that applies evenly to those who have other views, the ethical component needs to be stated in no uncertain terms.
You have just said in a fancy way that you disagree. Nothing more. Your argument is based on the fact that you believe ethics is only at the heart of polyamorous relationships and no other ones. I disagree and most monogamous people would also disagree with you. And again, abuse happens in all of these relationship structures. If you think abusive relationships can't be poly, then they can't be mono either.
Again, comparing monogamy to polyamory is not logically coherent as they both have different qualifiers.
In your opinion. Because you have declared that some people who wrote about polyamory and are considered authorities in the matter have written about how it's good to practice it ethically, you have decided that it must be in the definition as differentiated from monogamous relationships (despite the large volumes of books written by authorities on monogamous relationships saying similar things). Your definition only works if you are comfortable holding a double standard between the two relationship models. (Though I do find that people who think poly relationships are inherently superior to monogamous relationships tend to be pretty comfortable with that double standard.)
Monogamy limits its definition strictly to the number of partners involved. Polyamory carries with it an ethical component, and therefore one could rightly claim that unethical behavior with respect to the interpersonal aspect of a romantic relationship would disqualify it as as polyamorous.
Again, *in your opinion*. As someone who likes to point out other parties "hand waving", I'd say that this is a great example of it. You have only addressed the logical inconsistencies of your argument by restating your flawed premise.
And why is this important? Because abuse does happen in polyamorous relationships (more than one thinks) and it is dangerous and enabling of that abuse look at polyamorous relationships with the assumption that they are being ethical.