"Sec. 9. (I) The importation of negroes of the African race from any foreign country other than the slaveholding States or Territories of the United States of America, is hereby forbidden; and Congress is required to pass such laws as shall effectually prevent the same."
The British had banned the slave trade in 1807, and were working hard to get all countries to ban it. They even kept a squadron of the Royal Navy off the coast of Africa to try to catch slave ships, sieze the ships and free the slaves.
The Confederacy wanted to entice Britain to be an ally, or at least a friendly neutral. This was hard enough, given that Britain was against slavery (It had been banned in the British Empire since 1833). Britain was even more committed to banning the slave trade than to banning slavery, and if the Confederacy had encouraged a resumption of the slave trade (which had been banned in the USA since 1807) all chances of friendly relations with Britain would have been lost.
Well, the federal Constitution had outlawed the slave trade (importation) in 1807, iirc. The Virginia economy in particular at the start of the war was making a ton of money selling excess slaves further south, so keeping new imports out could be seen as a logical protectionist move.