Those pictures were taken in the 1920’s when the Ku Klux Klan was at its height of political power, if I remember correctly.
That is my understanding as well. At that time, that group was considered mainstream and acceptable. It reflects a period when the whole country was pretty much racist, and unapologetically so.
Nobody associates the American flag with them just because they co-opted it for their own purposes. Nor should they do the same for the Battle flag of the army of Northern Virginia.