I think it's fairly common with subsidized pay as you go phones (at least in the UK), since the network has to earn back the subsidy somehow. With phones on a contract, it's not necessary since you're locked in to the contract, but even so, it at least used to be the rule that the phone would be locked to the network, but they would provide the unlock code once your contract was over.
EDIT: Yep, turns out all the major UK networks (except Three) lock phones: https://www.ofcom.org.uk/phones-telecoms-and-internet/advice...
https://www.canada.ca/en/radio-television-telecommunications...