For decades there has been endless policy wrangling over whether “unlocking your phone” (removing restrictions allowing you to take the device with you to another carrier) should be allowed. Giant carriers have generally supported onerous phone locks because it hampers competition by making it harder to switch providers. Consumer rights groups and the public broadly support unlocked devices.
There are cases other than what you mentioned where the carrier refuses to unlock. During 2021, AT&T forced a phase out of 3G and provided allowed customers to get a free upgrade phone in the process. As a prepay customer, I took the phone but jumped ship to a different carrier. I tried to get them to unlock the phone earlier this year and they refused. There was no contract/lease involved, but they didn’t like that I didn’t remain a customer with them for (I think it was) 180 days after that.