A Question for the Japanese Government

So Ms. Roxana Saberi, dual citizen of the United States of America and the Islamic Republic of Iran, has been convicted in a secret trial on charges of Espionage... the third set of charges leveled against her after her arrest in January. Harsh, and likely unreasonable treatment of a practicing journalist with credits like the BBC and NPR on her resume (likely unreasonable as no evidence what so ever has been released by Iran in regards to her case).

Her mother is Japanese.

If, as would have been usual, her mother saw to registering the child at birth in the koseki (family registry) of her mother's family line, then is she not presumed under Japanese Law to be a national of Japan as well?

After all, one doesn't need to have the family name "Fujimori" to be given proper and due consideration of nationality.

Do they?