Their focus is on slavery, one could say they were obsessed with it, and potential impacts to that.
And as a side note, your man Rhett was threatening secession in 1856 if Fremont had won. And the primary benefit, as Rhett saw it, had nothing to do with taxes and everything to do with a resumption in the slave trade. Secession would, as Rhett saw it, "make Territories, now free, slave Territories, and to acquire new territory into which to extend slavery – such as Cuba, North Eastern Mexico, &c – but we would re-open the African slave trade that every white man might have a chance to make himself owner of one or more negroes, and go with them and his household goods wherever opportunity beckoned enterprise.”
People are obsessed with their cash cow. Philosophers have long noted that people's opposition to or favoring of something depends on who's ox is getting gored.
but we would re-open the African slave trade that every white man might have a chance to make himself owner of one or more negroes, and go with them and his household goods wherever opportunity beckoned enterprise.”
Rhett has seemingly swallowed the whole "slavery is moral" thing hook line and sinker. A lot of people in the South at that time did. They were wrong, but such was the culture in which they emerged.
It doesn't make his economic arguments invalid though.
Slavery needed cheap land, in order to maintain its economic viability. A free man, working his own land, would be much more industrious than a slave who only worked as hard as he had to, and take every opportunity to goof off. As the country filled up, free men would drive up the price of land. If slaves were not economically viable, then their price would drop to nothing.