If the Axis had been dropping 1000+ tons of bombs on American industry every day, then maybe that would have been rational.
Similar if the Axis had large-scale resistance forces operating in America, able to sustain at-scale acts of industrial sabotage.
But the Axis already knew that America had a huge copper industry. With no way to affect that industry, at scale - long lists of American copper mines, refining facilities, factories, etc. were no more valuable to the Axis than collections of apple pie recipes.
Similar if the Axis had large-scale resistance forces operating in America, able to sustain at-scale acts of industrial sabotage.
But the Axis already knew that America had a huge copper industry. With no way to affect that industry, at scale - long lists of American copper mines, refining facilities, factories, etc. were no more valuable to the Axis than collections of apple pie recipes.