Most tribs can't raid, and are often obliged not to participate in raids, which includes picking off stragglers (something many find cheap).

Most tribs can't attack Imperials in city. Most imperials can't attack tribs in city. Tribs spend lots of time in city.

Outside fortress is prime hunting terrain for someone with chamo. Probably the best place in the mud. So some will hunt there. That doesn't mean that there's not more cooperation between the two cabals though.