1. Carrier locks prevent dual-SIM use where one SIM belongs to the locked carrier and the other does not. This is of no value for preventing unpaid phones from being taken overseas but has plenty of value for locking paying users in.

2. Carrier locks require a communication channel from the carrier to the lock database to the phone. With iPhones, for example, the whole mechanism is mediated by Apple. The same mechanism could absolutely prevent activation overseas — instead of having a “locked to carrier X” state and an “unlocked” state, have a “locked and cannot be used for non-emergency purposes” state, an “unlocked until time T” state, and a “fully unlocked” state. And this would even be simpler: it doesn’t require any integration with the baseband processor, so it could be implemented straightforwardly on the AP.

(Note that iPhones already have a locked-to-an-iCloud-account state and can be remotely locked out, which implements 95% of this.)

So, in summary, I do not believe that the current carrier lock scheme is honestly or competently designed as a theft-of-unpaid-phones prevention measure.