In follow up to Sayaka's post (immediately prior to this one) I saw this article on the BBC today.
At first glance, the issue of repatriation of Korean individuals stranded in Russia for decades would seem to be a simple one. They were Koreans, forcibly removed and sent there. On the other hand, as the article points out, these people have lived there for decades, have had families who were therefore born on Russian soil and are Russian citizens. However, it does seem that many would like to return to Korea despite leaving their families behind on Sakhalin.
The question of why this was a non-issue for so long is also interesting. The article cites Cold War tensions, which make sense. I don't really know how the US was perceived by Koreans during the Cold War, but I'd be interested if anyone has ideas.