Rowan, I fully understand what you are saying, and appreciate the insights from a cultural point of view.
We have a saying here in the States which is equally repulsive to the one you sited, when thanking someone for doing a good turn..."that's mighty white of you." Lately, the trend in its use has become more sarcastic, it is used more often by people of color, and used to emphasize negative white traits. Tit for tat, I suppose.
We still disagree on the "descended from" issue. I happen to be descended from Scotch-Irish, Swedish, and Cherokee forebears, and look on all of my family history with interest, if not always pride. Yes, my mother's family is rooted in Southern Kentucky, Tennessee, and Virginia, and while my Great Grandfather x 3 Jacob Elder apparently held two black slaves, his grandson David rode in the Union Cavalry to end slavery. He is the same man who wedded a Cherokee woman after returning from the Civil War, a woman that was mentioned only in whispers, even into my Mom's generation. But I am descended from him and am proud of his spirit, and of my Cherokee heritage as well. My hope is that, in time, the blood of all of those who were maltreated will be celebrated by their descendants.