I can only say that when I read the books as a child, it was clear to me that Ma had a fear and dislike of native Americans that was not shared by Laura and Pa, and that Laura found this to be a flaw. It was also clear to me that the Ingalls family had trespassed on Native treaty lands. I was also well aware as a child of the history of mistreatment of Natives, broken treaties, etc. Maybe I was weird or unusual, but I usually sympathized with the Indians, not the cowboys. 
I remember my mother saying to me, re Thanksgiving, that Indians had no reason to celebrate it. It made me think.
We read favorite children’s books to our son when he was little that were rife with ethnocentricity, making fun of religions other than Christianity, etc, yet had wonderful content: VM Hillyer’s “A Child’s History of the World” and “A Child’s Geography of the World”. But when reading aloud, we stopped and pointed out such attitudes, and discussed them. Critical thinking can and should start before kindergarten
I’d be a heck of a lot more worried about the distortions of history peddled by Disney movies such as Pocahontas, because a lot of little kids swallow that stuff unquestioningly.
I would also point out that if books were to be condemned because of reflections of realistic sexist, classist and ethnically prejudiced attitudes, there would be virtually nothing on the shelves.
I think this group has made fools of themselves.