[4CJ] I guess that explains why the South protested over the amount of the tariffs almost since our founding? </sarcasm>
Two answers suggest themselves to me:
A) Are the economic historians working with good data and without an agenda? Remember that the Marxist historians at Columbia have been beavering away for years, trying to recenter the Civil War as a moral issue and as a didactic political drama of vanguard liberation, one of Marxist-Leninism's favorite (distorting) prisms. It would help their case if the tariff issue could be made to go away, and if slavery could be isolated as the solitary issue on the national bill of disagreements. Their politically-loaded mantra for years has been "it was all about slavery."
B) The data may or may not support a subtle argument that the tariff did not redound to the disadvantage of the South. However, the majority of Southerners thought that it did, and they thought so very firmly. As evidence, I would bring in (and cite if asked) the speeches of Robert Toombs and Alexander Stephens, the former a Democrat and the latter a Whig and a defender of the Whig program (Henry Clay's "American System"), debating secession and the iniquities of the tariff before the governor, legislature, and leading citizens of Georgia, as Georgia ruminated on the secession question after South Carolina, Toombs's home State, had already left the Union.
Stephens labored to make the argument LS does, but Toombs carried the day. Georgia, convinced she had nothing to look forward to but maltreatment and financial ruin from a Union run by Lincoln's sectionally-based political machine, seceded.
How does a tariff hurt a subsistence farmer?
How does a tariff hurt a subsistence farmer in the South more than it does one in the North?
When the topic is the socioeconomic makeup of the South, we hear that the vast majority of the population was dirt farmers with no stake in slavery.
When the Confederate army is discussed we hear that almost all reb soldiers were not slave owners.
Yet when the tariff is discussed, "the South" becomes a land that was 100% populated with plantation owners growing cash crops who would be hurt by a tariff.