They already did that, didn't they?  http://www.theverge.com/2013/8/23/4651926/us-government-seized-5-million-from-bitcoin-behemoth-mt-gox

In any case, MtGOX is a Japanese company, so transferring Yen to Japanese banks is obviously much easier and faster than sending them to other countries.

For the same reason, they can be sued by a wronged Japanese client in Japan far more easily and cheaply than by a foreigner located in another country.