He isn't on Erebus.
Sabathiel entered Hell to lead the Bannor out of it. He lead them to a portal through which they could return to creation, but he did not accompany them through it. After returning to Erebus, the Bannor constructed the Halls of Sabathiel. However, they did not create the holy of holies of this temple; it does not exist in Erebus, but rather in a portion of Junil's Heaven, or some sort of plane in between. It is not clear whether the Hight Priests of the Order themselves actually leave creation in order to communicate wiht Sabathiel, or if everything that goes on in the inner sanctum is actually a vision.
Sabathiel cannot really control the Bannor, he can only give them orders which they may or may not choose to follow. His priests are not perfect, and have been known to misinterpret, or even willfully distort, his commands. Eventually, the Order was taken over by wicked priests acting in his name, but directly against his will. Since they were the only ones legally allowed to inter the inner chambers of the Halls of Sabathiel, and since Sabathiel is bound not to interfere directly, know one on the outside knew of their corruption, or of exactly when they turned away from their faith. Still, it is clear that they were quite corrupt by the time they were hunting down all true followers of Junil, including Valin.
Also keep in mind that the Compact doesn't forbid all interference in creation, only direct action. The Gods are free to fight through mortal servants (much to Cassiel's chagrin), and the power they are able to exercise in Erebus is proportional to the faith of their believers. Just as Bhall was able to rain down fire from heaven for no good reason when she fell (without violating the compact--although I imagine that her subsequent battle against her good archangel, Brigit, might have been a violation), Junil does have some freedom to act. It has been stated that 300 years into the Age of Rebirth, The Bannor Empire was the largest in the world and the Order religion was the most widespread; thus, Junil has more freedom to act than any of the gods (except those that just ignore the compact, I guess).