They just hate it when you remind them that their hands are as dirty as anyones when it comes to slavery.
Personally, most of my ancestors were still in Europe during the heyday of the slave trade, and once here were more likely to invest any capital they had in more midwestern farmland and a new plow. But I'm not going to argue that a good part of the shipping interests of New England were up to their necks in the slave trade. Obviously they were. I'm arguing 4CJ's claim that "millions" of yankees invested capital in the trade, a claim he refuses to back up.