In the numerous times I've seen this topic come up, I've never seen anyone disagree with the point you are making here. Yes, of course it's her choice to put that in her ad. Nobody is disputing that. I think what posters are saying is that the ad itself is racist, or if you prefer, the choice itself is racist. Yes, it's her choice to refuse Black customers, but it's still racist.
Pretend it's 1960. You're in Alabama. You walk past a restaurant. A sign hangs in the window: 'Whites Only'. The restaurant owner placed the sign there. It was his choice to do so. Is the sign racist?