Sometimes, anti-trade policies are pursued on purpose.
This example that I found in a biography of George Kennan displays the other, unintentional channel:
Then on June 6 Senator William Proxmire of Wisconsin, citing Tito's handling of the "nonaligned" conference, proposed an amendment to the foreign aid bill denying assistance in any form to Yugoslavia. This pleased his colleagues, who extended the ban to include Poland, and it passed by a vote of 57 to 24. Reminded that they had precluded agricultural exports, the senators then amended the amendment to allow these. The world's "greatest deliberative body," columnist James Reston fumed, had thereby insulted both countries, first by cutting off all aid, and then, as an afterthought, by making them "a dumping ground for farm surpluses."
... These congressional actions, Kennan was quoted as saying, reflected "appalling ignorance" about Yugoslavia and amounted to "the greatest windfall Soviet diplomacy could encounter in this area."