I think if you plop your city too close to the wildlands then yes they should attack your cities early on. If I recall correctly though the AI is not programmed to attack your cities directly, but if they are in the way of travel then your city will get attacked.

The point of the green border is to demonstrate a dratically different landscape when you enter the elemental lords domain. I agree that these monsters shouldn't wander too far from their borders, but the borders are not a stop all. Also, when you cross the border you get a cut-scene so that border works like a trigger.

Now elemental lords shouldn't leave that border, but the minions, I see no problem with them wandering out.