Because perception is everything and reality is nothing. Japan's neighbors will fear a nuclear Japan, regardless of whether they have reason to.
And the tiger shows his true stripes.
Huh?
Reality is reality, and fact is reality. To say that perception is reality is to admit that you are unburdened by facts, or even by rational thought itself.
Japan's neighbors are: China, Russia, Taiwan, Malaysia, Indonesia, the Philippines, N. and S. Korea, and Vietnam.
1) China, North Korea, and Vietnam are all Communist states. All are in direct military and trade competition with Japan. Of course they will be uncomfortable with a nuclear Japan; they would all be allied against her in any regional conflict. If the point you're attempting to make is that a country's defense policy should revolve around not upsetting your enemies...
Ditto Russia. While a trading partner with Japan, the Russians have never been exactly close with the Japanese. The Kurile islands (Japanese territory) are still occupied by Russia, as they have been since their invasion in WWII.
2)Malaysia has historically been a monarchy, but control has recently been placed in the hands of pro-capitalism Islamists. They are anti-Communist, and they have good relations with the Japanese.
3) Indonesia is also a Muslim country. They have very good trade relations with Japan and cooperate on a number of military, humanitarian and anti-terrorism fronts. They are anti-Communist, and have good relations and defense/weapons agreements with the US.
4) Taiwan has excellent relations with both the US and Japan, and has signed defense agreements with both countries. Are you honestly suggesting that Taiwan would side with China on the Japan nuclear issue?
5) The Philippines also enjoy excellent relations with Japan. They are trading partners, and cooperate in anti-terrorism activity. The Philippines have free elections, are staunchly anti-Communist, and have trade and security agreements with both Japan and the US.
6) S. Korea is a major trade partner with both the US and Japan, and has arms and security agreements with both countries. Although generally hand-wringers when it comes to the presence of US troops on the SK peninsula, they have (to date) loudly protested at plans to remove American forces. There are certainly many in the citizenry and government who fall for the media-savvy Chinese party line of "American oppression of the South Korean people", but when push comes to shove, who do think they will side with... the country that sent half a million troops to aid the North Koreans in crushing them during the 1950's, or the country that kept them from under the bootheel of Communism?
Additionally, Indonesia, Japan, Malaysia, and the Philippines are all allied against Chinese claims to the Spratly islands.
Japan's neighbors, the ones that count, will side with Japan.