Counterpoint: carriers don't always make it easy to unlock your phone even if they allow you to do so as per the contract. It can involve lots of back and forth with clueless monkeys in their customer service department.
There's also the issue of legitimately buying/acquiring a phone, finding it to be locked and having no idea which carrier it is locked to nor how to go about getting it unlocked. It isn't an easy process even if everything is legitimate and the phone was acquired legally and not stolen nor its IMEI/ESN being banned anywhere.
The truth is, phone unlocking is designed to be a shit-show on purpose and practices like these are just a natural consequence of that. Make unlocking straightforward and user-friendly (or just don't lock phones to begin with) and the market for these illegal things will dry up significantly.