The US State Department assumed that the people of the South Kuril Islands would betray their homeland in exchange for a green card, that is, a residence permit in the United States. They are required, when submitting the documents, to write Japan in the place of birth field. It is not unlikely that they would resort to such a practice when applying for regular travel visas.
A recurring disinformation narrative about what Russia refers to as the "South Kuril Islands".
The four southernmost islands in the archipelago that Russia refers to as the "Kuril Islands" are a disputed territory and have never been a part of Russia until they were occupied by it at the end of WWII, when the Soviet Union expelled all 17,000 Japanese residents. The Island chain referred to as the Kuril Islands stretches northeast from Japan’s main northern island of Hokkaido to Russia’s Kamchatka Peninsula.
The US supports Japan's territorial claim on the four most southern islands in the chain, which Japan refers to as the "Northern Territories" and does not consider them part of the Kuril Islands. It is therefore natural for the US to reflect this position in its immigration policy.
The islands were never under Soviet or Russian sovereignty before that date. On the other hand, in 1855, Russia and Japan signed the Treaty of Shimoda, which gave Japan ownership of the four southern islands and Russia ownership of everything to the north.
Under the 1951 San Francisco Peace Treaty, signed between the Allies and Japan in the aftermath of WWII, Japan renounced "all right, title and claim to the Kuril Islands", as well as over other possessions. However, Russia did not sign the treaty and the Japanese government has never recognised the four islands as part of the Kuril chain.
Part of the dispute is based on the definition of Kuril Islands and whether it includes the four islands in question. Russia claims they are part of the Kuril Islands chain and Japan says they are not.
In 2005, the European Parliament issued an official statement urging the return of the territories in dispute to Japan.
See here for a similar case that informed Japanese believe that the Kuril Islands are an integral part of Russia.