These are some of the biggest causes of the war, along with the vastly different cultures between the two nations. Because they really were separate nations, and the North was just as foreign as Britain to the South, so why shouldn't they set their own economic policies? If the South had separated, there would no longer be any reason to condemn free trade with Britain any more than trade with the North.
I guess you could make that case about any separatist movement. The point is, nation states are formed through projection of power.
Emperor Constantine certainly didn't nicely ask whether regions wanted to remain within the Empire, and Theodosius also didn't run a referendum on whether Christianity should be accepted. Bismarck didn't beat the Habsburgers in the primaries.
One of the most powerful ideas behind the founding of the US was to create a self-sufficient nation that can practice self-governance. Being able to do that is contingent on geographical factors. Sure, they could have let the South secede and left it at that, but in that would have been opening themselves up geographically to a confederacy in the South that is strongly connected to the former colonial power (the Southern Elite was Anglophile and Jewish, to a larger degree than people like to admit nowadays).
An english-speaking free trade nation would once again have turned into an outpost of the then still very powerful British Empire, and the Unionists didn't want that. You can make an ideological argument that it would have been "more democratic" or stuff like that, but I don't really care about that. We've played the democracy game and it doesn't necessarily produce good outcomes.
I know, the first layer of revisionism is that ackshually, Lincoln didn't like black people and the South had a right to secede, but you have to go deeper.
The idea that the South would have been this Jeffersonian utopia upon secession is ridiculous, and it is solely rooted in viewing the past through the lens of today's political alignments. By the same token, both the North and the South would have looked a lot differently if they hadn't let all the Eastern Jews in at the turn of the century (which the South would probably have done en masse without anybody even asking, because they were just so Jewish and because they were susceptible to all the classic "muh competition" free trade arguments.
The South was pro free trade, pro France and Britain, they were happy to have a retarded peasant population and were also very, very Jewish and Masonic.
Lincoln's core idea regarding the Confederacy was "Look, we've got a great thing going here and I won't let you break it because you would rather employ negroes and work rednecks to death than invest in improvements" with a subtext of "I don't trust those Jewish masons and their army of ni**ers".
The true redpill on the Civil War is that libshids are right, but not in the way that they think. The rights of Black were neither here nor there, Lincoln would have preferred to get them off the land.