[GoNative]

From what I've read the US is actually one of the few countries that does uphold custody agreements from other countries (who are signatory to the child abduction convention) and will prosecute it's own citizens that have abducted children from those countries.

The problem in Japanese cases is that generally the Japanese courts do not get involved in custody issues and do not make rulings on custody. So there is very little say a court in the US can rule on as there is no legally binding custody agreement. Even if a child has been abducted from Japan back to the US (this is rare compared to the other way around) then because Japan is not signatory to the convention then the US is not bound in any way to uphold any agreement that may be in place.

Japan signing the agreement will protect it's own citizens as well.