EA does not have to simulate the cities now, each cities are already simulated/view only by the owner of that cities. EA only have to relay information between cities.
But they would have to do it if you can actually control other cities in the regions. If you play one city and then switch to another city for, say, 1 game year, what's going to happen to your previous city? Does its time stop? Would AI have to take good care of all cities?
