I never understood the economics of cell phone carrier locking. So you buy a phone that is meant to be paid off over a 3 year contract. Then you unlock it and

I never understood the economics of cell phone carrier locking. So you buy a phone that is meant to be paid off over a 3 year contract. Then you unlock it and sell it. That doesn't invalidate the contract, does it? Is it that easy to walk away from such contracts?