You don't seem to realise how big a deal it would be to the bitcoin community the day that a judge actually says, "it's clear that FOOCORP has defrauded you of X amount of these bitcoins so FOOCORP shall now return those X bitcoins or the amount of Y USD which is the equivalent as of today"
Until that actually happens, it seems far from clear to me that "they are equivalent" in the eyes of the law of the US, let alone Japan, and I can fully understand anyone who believes they are legally safer to argue over USD rather than BTC. There is a very long list of massive thefts of BTC and a list of precisely zero entries of people who got legal remedy for that.
In the case of MtGox my personal belief is that if they walk away with the funds, no one will get anything back, in any currency, except maybe Yen.