I googled “Society for Requiring the Admission of Historic Responsibility of the US Government for Dropping Atomic Bombs on Japan” and the only place it showed up was on this thread. I would have expected at least one other reference if they really existed. So it is neither the entire nation of Japan nor the government. At most it is a couple of Japanese kooks., or maybe even it was just made up by the Brits who wrote the source web article.
“Society for Requiring the Admission of Historic Responsibility of the US Government for Dropping Atomic Bombs on Japan”
It could be that the only sites mentioning them are Japanese-language sites, or that this article’s translation of the name from the Japanese differs from the usual one.