that has nothing to do with unlocking the phone though.
Let us look at a lease of a car... Sure you don't own it, but you are effectively the 100% caretaker of it and financial responsibility for it.
If you don't change the oil in it for 2 years... or run the tires down to the cords... Or never clean out the yuck from the kids... Wait, you just turn it in after 2 years.
With a phone, once the contract ends... its a "rent to own" type of thing... so you are supposedly the owner. If you pay the Early Termination Fee... what the flip do they care anymore.