Perhaps because Watcher is dedicated to destroying the cities?
I could see a very temporary alliance against some great threat that was about to destroy the world... but beyond that, I couldn't ever see Watcher and Trib working together.
This man is one hundred percent correct.
As to the original idea, a no-pk area was in consideration once. Vestiges of it still exist, but it will not be implemented. The reasoning is that under an active Tribunal the cities become very safe. They are not one hundred percent safe, but they are very safe. When the Tribunal is not strong, it means the empire is not strong. When the empire is not strong, chaos runs rampant in the cities. It is a cycle - an ebb and flow if you will.