Frankly I dont' think conservatives in general really grok how bad slavery and racism it engendered was and is.
What the hell are you saying here?
Grok: .... to understand intuitively or by empathy; to establish rapport with" and "to empathise or communicate sympathetically (with);
http://en.wikipedia.org/wiki/GrokThere's being told something and maybe getting it intellectually.
And there's having a deep understanding of it. The two are not the same.
To many conservatives, slavery was just this thing that happened in the past, with few to no consequences beyond providing Liberals something to complain about. They don't see a connection between it and racism, or how it has affected our history beyond 1865 or the damage it did to generations. Intellectually they know slavery happened and racism exists but they don't see the logical consequences of that fact.
So slavery, like other forms of social injustice and oppression is just this political football, a way to score rhetorical points and evoke an emotional reaction. They really don't understand this as offensive and beyond the pale.
Admittedly that is a rather generous interpretation.
It's also possible they simply don't' give a fuck if they look like assholes.