Well, if the first topic is right, this should not be possible: http://www.reddit.com/r/SimCity/comments/1d77t7/mod_pack_build_outside_city_box_simroller/
If it's not possible, how it happened?
Bigger towns (not cities, this is SimTown, remember?) are a simple question of data storage in my mind.
Also, they have many workarounds if they wanted add this, like being possible to buy expansion borders to your town, where you could go visit and build.
I really hate how near my industry is from my commercial and residential area, specially because of pollution and other things.
Plus, this is looking like an aquarium of ants, and not a town.