http://www.japantoday.com/e/?content=news&cat=9&id=294190
On basis that it violated State and Religion. Plaintiffs said that as Koizumi attended these rituals as a member of state, that Koizumi violated their freedom of conscience as they did not view shinto-ism and veneration of war dead as their own.
After this successful case, there are 5 other similar case proceedings in 5 other districts around Japan.
I think this is a good thing. Koizumi's always been a radical and I support his free thinking ways. But his visits to these shrines have not agreed with me well. Partly b/c being Chinese myself I feel he should be more sensitive, and partly b/c this furthers politically isolates Japan from her neighbours.
"Despite persistent opposition from the public and even from the ruling Liberal Democratic Party, the premier visited the shrine, which is not necessarily an appropriate place to honor war dead, based on political motivations," Presiding Judge Kiyonaga Kamegawa said.