Yeah, Tsunade sent a squad of Genin (and a Chuunin) after the holder of one of Konoha's biggest secrets.

Why wouldn't Danzou try to kill off Sasuke? From his perspective -- and that of most of Konoha -- Sasuke is a missing-nin. Danzou isn't ANBU, and wouldn't know about Super-Secret ANBU missions. He could potentially have spies in ANBU, but they'd have to know about a secret mission going on since the Chuunin exam, when only the Third and his most trusted Jounin knew Orochimaru was in the village.