If the flag of an armed enemy of the U.S. is allowed to fly over government buildings, then it implies that slavery, or at least the threat of slavery, is sanctioned by that government and can still legally exist.
“Every time I write a new novel about something sombre and sobering and terrible I think, 'oh Lord, they're not going to want to go here'. But they do. Readers of fiction read, I think, for a deeper embrace of the world, of reality. And that's brave.”