Maybe that's the Oathpact. Odium agreed that in exchange for being able to subject the Heralds to everlasting pain, the Heralds get to come back and attempt to prevent the same thing from happening to mankind. They eventually got sick of it (see Prelude), and they gave up. They hoped that keeping one of them bound would be enough to keep the Oathpact, and they figured that the Radiants would do a good enough job protecting them.
EDIT- Except, there has to be a time limit in there. Because it seems like there's another reason why they follow the Oathpact, a reason why Odium isn't constantly attacking.