While what you say is true, don't forget that the Dunhams were not the Nelsons (Ozzie and Harriett). They were far from being the typical middle-class, white, suburban or rural American family of the day. Stanley Ann wasn't a rebel child raised by conventional parents; her father, especially, seems to have been quite unusual in his ideas and lifestyle.
It would not surprise me if a man who was drinking buddies with a Black Communist Party member and who thought it was appropriate to take his young grandson to bars, would have encouraged his daughter to have had relationships with men of other races.
You make a good point. Her parents had some STRANGE ideas. I had heard that before, but it bears repeating.