no law that I am aware of. But is most likely a carry-over from previous centuries when countries in economic opposition to each other wanted it to be known where their products come from as a nationalist issue.
it's also likely that printed materials of any kind from 'other nations' (depending on what nation you are of course) were subject to a tariff, to induce companies to source products from their own nations and to prevent other nations from dumping materials on foreign markets, to prevent downward pressure on wholesale prices of material (like China selling steel) and causing home nation countries to go out of business and cost jobs.
