Because people did think those things in the past? Blackface "minstrelsy" was one considered a perfectly acceptable form of entertainment. That doesn't mean it should still be. But it also doesn't mean it's okay to ignore the fact it once was.

Those depictions of the time show what people thought at the time. I can see cutting parts out that have no relevance to the plot line, like with the Lofting example above, but the whole point of the Little House books is to show the "colonization" of the prairie. The best way to show any "less enlightened time" is to show it the way it was, warts and all. Otherwise we fall victim to the idea the past was somehow perfect.