This is due to Kusuko being FUJIWARA no Tanetsugu's daughter, but it is also said that the Emperor intended to have correct proof as to Prince Sawara's inability to become the crown prince, and the Emperor's imperial succession (however, the article was again removed by Emperor Saga).
これは薬子が藤原種継の娘であったこともあるが、早良親王廃太子と自分の皇位継承の正当性を示す目的があったとされている（後に嵯峨天皇によって再度削除されることになる）。