Apple get a cut of the contract. This sweetens the deal for them so they can offer the iPhone at a reduced price. There is room here to interpret that at subsidization. An unlocked iPhone is highly likely to cost more.
As long as we have that clear then this may work out ok for the consumer but the device price won't stay at current levels if its unlocked.
You might want to open your eyes. The price the consumer pays for an iPhone is more than the cost to manufacture it. So Apple makes money by selling just the phone. Then the carrier is giving Apple the subsidization, which is pure profit to Apple. Then Apple is also getting a piece of the pie every month. So having a piece of the contract so they can offer the iPhone at a reduced price is not accurate. They are making money at every turn.