They should not earn money if people don't go to the city. A city without people is a dead city, a dead city has no money. That way they should actually do something about "town control", and make people want to go to the city.
example: -Klamath is taken by the X Gang, they kill everyone that enters, that way they get no money at all, even they could get a penalty. People stops going to the city.
-The Den is taken by the Y Gang, they have players watching the streets, they have a trader with good prices, and they have contests giving away metal armors. A lot of people starts coming to Den, and the Y Gang gets a lot of caps.
That's how we can make Town Control useful for everyone. Of course they can have they're rules, but they wont make a rule that makes them lose money...