How would that threaten the North? Imported goods taxed by the Confederacy and then again by the U.S. would be less competative than goods imported into the U.S. directly and taxed only once.
My first thought was to explain it to you. Then I thought, "she doesn't really care and she won't remember it or acknowledge it even if you went to the bother. "
I could give you newspaper quotes from northern newspapers that would explain it to you, but you've probably seen them before, and don't remember them.
I could mention the threats northern port cities made to abolish federal tariffs if the southern states did so.
I could mention the lament that they could not possibly patrol the long porous border with the South.
I'm finally left with the idea that if you objectively understood what was being discussed, I wouldn't need to explain it to you.
The tariffs were unenforceable in the interior and along the Mississippi watershed. They only worked because of port city choke points.
The foreign products would be sold domestically by smugglers and the higher Union tariffs would not be paid on them.
And everyone in that era who dealt with trade knew this.