Well overdue, people receive a subsidy in exchange for entering a service contract. Whether a phone is locked or unlocked has no impact on a customer's obligation to adhere to the contract.  If they want out of their contract they have to repay the balance of the subsidy, otherwise they make their required monthly payments for the duration of the contract.

The carrier lock is about cashing in on their customers when they travel abroad and use their devices.  The lock prevents the customer from using a local carrier (swap simm cards) when traveling.

Domestic carriers can either offer more competitive roaming rates or let foreign telcos have the revenue.