I assume the main reason is so they can't just fly through the nomad worlds unpunished.
Imagine Outcasts loitering around in front of Hurr Durrikun *shudder*
Outcasts were first to encounter Nomads, due to proximity to Nom homeworlds. At the beggining, Maltese ancient burial ground coincided with jumphole to Nomad worlds causing "guardiands" to appear, seemingly protecting their dead as well. Much later, there was more and more interaction between the two. OC government decided to enforce peace (and avoid another front) with aliens, who viewed them as rather interesting (both use cardamine through for different purposes).
Nomads quickly used opportunity and established cults, swaying more and more humans. It escalated enough to get The Order involved as cults became more and more apparent and OC frequently assisted Nomads in combat.
Of course, not every OC has to like it and no one but those directy involved with Nomads can simply stroll to Iota without permission. Rephack is therefore justified IMO.