I don't think they're trying to justify what people did then, but just trying to give the slave owners viewpoint. That not all slaveowners were monsters, some saw themselves as good Christians and did treat their slaves fairly by their standards.
This might explain the not uncommon incidents of slaves hiding and protecting their owners property after the owners had fled and Union troops arrived. This behavior doesn't make any sense from the soon to be freed slaves viewpoint if the owners had treated them pooorly.
It was a strange system that warped peoples decisions and perceptions. The only upside is that this country ended up with a lot of great contributions from citizens that otherwise would have never immigrated here.
The "Stockholm Syndrome" is also a well-known psychological phenomenom of some hostages beginning to identify with their captors after long periods of captivity and dependence on them. That doesn't make the hostage-taking any less of a crime or imply that some hostages were really quite content being hostages.