I have no expertise in this area, but here are my comments.
The IRS instructions for Form 1099-DIV box 8 (for the company that is filling out the 1099-DIV) say "Enter the name of the foreign country or U.S. possession for which the foreign tax was paid and reported in box 7." I would call the brokerage again and read that sentence to them. Point out to them that the United States is not a "foreign country or U.S. possession," and that tax paid to the United States is not foreign tax. Ask them to either issue a corrected 1099-DIV, or to cite the law, regulation, or IRS source that allows "United States" to be entered in box 8. Follow up with an email or letter saying the same thing.
They will probably still refuse to correct it. At that point I would just give up and select Japan as the country that the dividends are from, even though the 1099-DIV says United States, and move on.
