Why would anyone have weddings at plantations considering their dark history?

I think it is possible that they are clueless to the reality of the events the took place there. They may know the stories, but they don't really understand them. I think of it as the difference between an education and an experience. Easy to overlook/ forget the education, but you cant ever forget the experience. They don't connect with the experience.
As AngelUnforgiven said, they have no empathy.