I think the issue is a separation leading to an idealized conception not closely conforming to reality. Pius was separated by distance, contemporary reb admirers are separated by time and/or regional defensiveness. The details of the Deep South civilization were unpleasant, unnatural and oppressed by a constant fear of racial conflict and insurrection. Not a real agrarian ideal except for those raking in the big bucks on slave labor.
Absolutely. In addition, the concept of pure chattel slavery was probably not what the Pope had in mind. Slavery had died out in Europe long before and had only been revived in the course of ransoming the hostages that the Muslims had taken - and the Muslims did practice pure chattel slavery, where the slave was considered simply a piece of property. However, in the Catholic world, the slave had certain rights (religious instruction, baptism, marriage, the right to own property, buy his freedom, not to be sold without his family, etc.). This certainly was not always respected - the Portuguese completely ignored these requirements, for example - but theoretically, from the Pope’s point of view, slavery would have been more like a cross between serfdom and long-term indentured servitude.
I guess I can understand it better in the case of somebody who’d never actually seen the US system in operation. The thing that always amazes me are some of the defenders of the South who declare that things were just great on the plantations and the master was really solicitous of the slaves and it was all just one big happy family. I actually heard an elderly docent in a museum say this - and the museum was in South Carolina, of all places, where the slaves on the rice and cotton plantations received such notoriously awful and brutal treatment that it even led to the famous Stono Rebellion. The slaves captured after the rebellion was put down were beheaded and their heads were put on stakes along the road as a warning.
You’d think that would be enough to make people see the reality of slavery, but as you say, there’s a lot of regional defensiveness and of course the rose-colored glasses of time. Naturally, nobody wants to think that they are descendants of people who treated other human beings like this.