I mean, when the rule was written (I'm recalling from memories a year ago, correct me if I am wrong
@Aeronwen ) it just was designed like this to not create confusion. Nobody cared about the rule up to now and I'd like to call out RM for ****posting in saying this rule was created DM biased.
On the other hand Outskirts could be considered an open map with Omor and Trading Post standing company to that. Echerion and Xauna can be considered closed maps on the other hand so by removing one of the open maps and reserve it for the tie breaker, you can just go and ban Xauna or Echerion to force an open map if you want to play an open map. If you go however and pick Xauna and the other side picks Echerion then this is on you, since it's impossible both open maps get banned unless both teams ban one of them.
Since the tie breaker was never used so far (and I don't expect use of it in the future) the admins could go and appoint Omor as tie breaker to bring back Outskirts into the rotation.