The Japanese were looked down upon by the US for quite some time, same with Southeast Asia in general. The Immigration Act of 1924, and the others before it which excluded Asian immigrants. The internal pressure the Executive put on the Judicial regarding Executive Order 9066 in Hirabayashi v US, and further Korematsu v US, including the falsification of evidence by the Executive. Hell, the US levelled virtually every urban region in Japan. I doubt America, especially under Roosevelt, cared deeply for the atrocities taking place in China. They just didn't want Japan to threaten nearby Allied territories.