You're ignoring the fact that when the US was a developing nation, it completely ignored the rights of foreign authors for a very long time. The Copyright Act of 1790 specifically stated:
"And be it further enacted, That nothing in this act shall be construed to extend to prohibit the importation or vending, Reprinting or publishing within the United States, of any map, chart, book or books, written, printed, or published by any person not a citizen of the United States, in foreign parts or places without the jurisdiction of the United States."
Henry G. Henn, "The Quest for International Copyright Protection" (1954):
"The United States had been among the most parochial of nations so far as copyright protection for published works is concerned. For over a hundred years, this nation not only denied copyright protection to published works by foreign authors . . . but appeared to encourage the piracy of such works."