On a lot of prepaid devices such as those from Kyocera for companies like Boost, the limitations are almost all in software configuration, because that's cheap and easy to do rather than rolling your own baseband configuration.

For years, carrier lock on iOS devices was simply a software switch. In a lot of devices, still, if you have an unlocked boot loader you can run patched baseband firmware that doesn't care that it hasn't been told the magic numbers to unlock itself.