Actually, that was very specific to the people I know who fly it. We're a 4H family and a certain amount of "redneck" is unavoidable in those circles in rural Michigan, so I've had a few friendly conversations (seriously, actually friendly!) about why someone born and raised a thousand miles and a century and a half away from the Confederacy would choose to fly that flag on their truck/camper, and that's the explanation I've gotten most often. And both the Confederate flag and Betsy Ross flag are popular with people who identify with/support the worldview of the "3%ers", which is alarming in a different way but not particularly racist.
I agree. It amazes me how readily some on this board challenge the existence of that kind of casual racism. I see and hear it around me fairly often. I think that comes from living in an area with little diversity and little social penalty for holding/expressing racist views out loud. Advocating violence would cross a line, but racism couched in "jokes" using offensive language is pretty acceptable still.