Perhaps, it's because there is only little delay with Japan domestic withdrawal, and they are all busying making arbitrage profit than to go knock on any door

Oh come on, if that were the case the price difference would have already been reduced to zero, which isn't the case.
True. Current Ask price of MtGox JPY is ~10590 = $108.12. So still 10% arbitrage between MtGoxJPY and Bitstamp USD.
Assuming 0.55% currency exchange cost this loop still very profitable
USD --> Bitstamp BTC --> MtGox BTC --> JPY ---> USD
But only one person has complained of a delay on JPY transfers despite many requests for comments from people using JPY withdrawal method.
Even then his complaint was that it took 1 week to arrive, not the 4-12 weeks for Sepa/Wire
https://bitcointalk.org/index.php?topic=179586.msg2939409#msg2939409If you can get your money within a week doesn't make much sense that there is such a price gap. I don't get it.
The transfer actualy only took one day, I had mispelt my name in Japanese.