Only in your imagination were they not. That's exactly how it worked. You think those ships sailed back across the Atlantic empty? That would have been incredibly wasteful.
James Spence of London explained the effects of such a high tariff on the Southern economy (1859):
"This system of protecting Northern manufactures, has an injurious influence, beyond the effect immediately apparent. It is doubly injurious to the Southern States, in raising what they have to buy, and lowering what they have to sell. They are the exporters of the Union, and require that other countries shall take their productions. But other countries will have difficulty in taking them, unless permitted to pay for them in the commodities which are their only means of payment. They are willing to receive cotton, and to pay for it in iron, earthenware, woollens. But if by extravagant duties, these be prohibited from entering the Union, or greatly restricted, the effect must needs be, to restrict the power to buy the products of the South. Our imports of Southern productions, have nearly reached thirty millions sterling a year. Suppose the North to succeed in the object of its desire, and to exclude our manufactures altogether, with what are we to pay? It is plainly impossible for any country to export largely, unless it be willing also, to import largely. Should the Union be restored, and its commerce be conducted under the present tariff, the balance of trade against us must become so great, as either to derange our monetary system, or compel us to restrict our purchases from those, who practically exclude other payment than gold. With the rate of exchange constantly depressed, the South would receive an actual money payment, much below the current value of its products. We should be driven to other markets for our supplies, and thus the exclusion of our manufactures by the North, would result in a compulsory exclusion, on our part, of the products of the South."
This is exactly what happened during the Tariff of Abomination. Cotton sales declined by 50% and the price decline considerably as well.
"This is a consideration of no importance to the Northern manufacturer, whose only thought is the immediate profit he may obtain, by shutting out competition. It may be, however, of very extreme importance to others — to those who have products they are anxious to sell to us, who are desirous to receive in payment, the very goods we wish to dispose of, and yet are debarred from this. Is there not something of the nature of commercial slavery, in the fetters of a system that prevents it? If we consider the terms of the compact, and the gigantic magnitude of Southern trade, it becomes amazing, that even the attempt should be made, to deal with it in such a manner as this."
And the North just let England take their wheat, corn, rye, timber, fish, beef, wool, furs, and all of their other exports. According to you, the South was the only exporter and cotton was the only export. Again, you are full of BS.