If a carrier cannot trust you (especially due to bad credit history), they can't sell you the phone you want if you don't pay outright, and many people aren't able to do that. SIM locks are basically "repossessions but for phones." Nobody argues in good faith that banks shouldn't be allowed to evict you if you don't pay your mortgage, and that's effectively what a "no SIM locks" policy does for phones.
To make things even worse, it's far too easy for fraudsters to buy a phone using a stolen identity, never pay a single installment, and instantly sell it on the second-hand market. For SIM-locked phones, the carrier can brick the phone until it is fully paid of, so that kind of fraud is not possible.
Sim locks do enable some abuse, particularly around roaming fees, as you're not allowed to swap your SIM when leaving the country. However, They're a blunt tool unsuited to the 21st century internet-enabled smartphones. One could imagine a mechanism where the carrier is still allowed to brick the phone in case of fraud, but there's no actual SIM lock.
If a carrier cannot trust you (especially due to bad credit history), they can't sell you the phone you want if you don't pay outright, and many people aren't able to do that. SIM locks are basically "repossessions but for phones." Nobody argues in good faith that banks shouldn't be allowed to evict you if you don't pay your mortgage, and that's effectively what a "no SIM locks" policy does for phones.
To make things even worse, it's far too easy for fraudsters to buy a phone using a stolen identity, never pay a single installment, and instantly sell it on the second-hand market. For SIM-locked phones, the carrier can brick the phone until it is fully paid of, so that kind of fraud is not possible.
Sim locks do enable some abuse, particularly around roaming fees, as you're not allowed to swap your SIM when leaving the country. However, They're a blunt tool unsuited to the 21st century internet-enabled smartphones. One could imagine a mechanism where the carrier is still allowed to brick the phone in case of fraud, but there's no actual SIM lock.