There was some logic behind this:
World war ii - Embargoes and Sanctions
"But while Roosevelt turned from sanctions toward measures of positive aid to Europe, in Asia his administration moved toward a more pointed use of embargoes against Japan. Japan relied heavily upon American oil and metals to supply its war effort in China. Any threat to stop those exports would have a significant impact on Japanese plans. The swing of public opinion and the revision of the Neutrality Act in 1939 allowed Roosevelt to take some action on behalf of China. So, in May 1939 the United States notified Japan that it was withdrawing from the 1911 Treaty of Commerce. According to terms of the treaty, in six months the United States would be free to limit or terminate exports to Japan. Roosevelt hoped this would give the Japanese pause, but Japan continued its war in China.
The U.S. government hesitated to implement sanctions for fear that they would drive Japan to replace the embargoed items by invading new sources of supply. This would most likely be Southeast Asia, where French, British, and Dutch
colonies were supplying those same vital materials to America's allies in Europe. The six-month period of grace passed, then a year, with no sanctions applied."
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The US was more worried about how things affected Europe than China. So the hammer didn't come down.
Again fate has subtle twists and turns. So when Japan did invade French IndoChina there was a reason the US paid notice.