Your reference to cars is interesting ... let me go off from there...
The price of a gallon of gas is, like most things, based on supply and DEMAND. The greater the demand, all things being equal, the higher the price. Now, not all vehicles get the same gas mileage so in truth, the folks that drive more fuel efficient cars tend to drive the price/gal down whereas those that drive gas guzzlers tend to drive the price of gas up. So, if everyone drove gas guzzlers the price/gal would be MUCH higher than it is whereas if everyone drove a Prius the price/gal would be a lot less. So, getting back to cell phone, the heavy users of data drive the cost of cell service up by increasing demand.
The unlimited plans will wind up being the stupidest idea any company ever had because it does encourage users to use more than they would otherwise use. If you go to an all-you-can-eat buffet and eat more than you need/should you, in fact, raise the price of that meal for everyone BECAUSE the restaurants cost is increased and to make a profit as all business must they have to pass that higher cost along to ALL there customers. This boils down to simple greed and we are almost all guilty of it from time-to-time, goodness knows Wallstreet lives by it and as a consequence we are dying from it...
Brian