That's mostly because the motivations don't matter, AND they're impossible to reliably determine from the outside. So it mostly is just baseless guesswork, and at worse a tool to cast doubts - effective only at shutting down discussions. You might've seen me troll Roland over something very similar, calling random people liars. It's just not useful.
Motivations don't matter might sound a little obnoxious, but they truly don't. Data, proposed problem and proposed correction do. This case is like trying to get the local restaurant to stop selling beef, because "red meat causes cancer". I don't need to question the motives of the person to show that "red meat causes cancer" is coming, through clickbait media, from WHO. WHO reviewed 14 studies on the topic to make the statement. 13 of those studies showed "no correlation", one showed "potential correlation" => WHO ended up relying on that one anomaly. Even science is done with "confidence intervals", the occasional anomaly is expected.
I don't think it would be too difficult to argue the merits, whether or not the person trying to stop the beef is a vegan. The data don't support the proposed problem, and the proposed correction would not alleviate the issue even if (people will just get their beef elsewhere).
In this case, the data are from "Native Advocate Groups" .. arguably not as "science based" as WHO... it is relatively easy to assume that they will not judge any case to be "free of Indian Oppression"; much like the counterpart business organization will not judge in favor of unfair tax advantages. But either of the claims could still be valid, and that is independent of the motivations from which they've been brought.
And while that sounds "trust the science"-y ... it's not. Physics and maths are producing reproducible science, everything else it engineering at best, and pure fairy tales aren't uncommon either. We will never have usable data for moral claims ... or human research in general.