Slavery is considered immoral by the current power structure. For the majority of human history it was not considered so. Many leftists consider wage labor to be a form of "wage slavery"; if they win, will supporting historical figures who fought for the right to employ workers be considered as immoral as supporting the confederacy? History suggests that yes, it will. Leftism is a ratchet. Oppose once hotly-contested ideas such as women's suffrage, gay rights or the civil rights act and you are kicked out of polite society. Concede that it is off-limits to even signal your support for a group (not even signalling your support for slavery itself), and you've taken the first step on the slippery slope towards agreeing with the leftist idea of progress itself, and conceding each of the points of their worldview, one by one.