Is Japan to blame ? I would say in effect no it isn't. Its altering the status quo that is the to blame.
As far as I know Japan is the 2nd largest contributor hence they have lobbied for SC membership. This approach has some merit. If you pay for it you should have some say in it. You know that if they US were the 2nd largest contributor and they were locked out of the SC the forces of rampant conservatism and finger pointing would be bemoaning the issue.
Morals and ethics aside this debate seems to be about the UN as a worthy international forum or not. Since the UN was in effect created by and for US interests its somewhat disturbing for neo-cons and or conservatives who currently disagree with the system put in place to claim it doesn't work. When it fact it has on many occasions worked quite well. Are there flaws ? certainly. name on human designed system that is without flaws...
the question was "is japan to blame" and the answer there is no. The chinese want to make it an issue, but for the other SC members I believe it isn't an issue.