Well, not only are you not using their service, you're using somebody else's service when theirs is available. As per your binding contract, you agree to only

Well, not only are you not using their service, you're using somebody else's service when theirs is available. As per your binding contract, you agree to only use their service in return for getting the phone at a subsidized price. Their incentive to charge you less for the handset (incurring a loss on the sale) is that you agree to guarantee doing business with them. And if that's a problem, unlocked phones are still always an option.