Earlier quoted context omitted.
How do they distinguish you from a phone thief? Is there some kind of check with the previous owner?
Locking the phone to a carrier is not an anti-theft mechanism. They're available in abundance on the used market with no special protections of any kind from the carriers, the only difference is that they sell for a fraction of the cost because you're locked in to the single carrier. Maybe you're thinking of the locking mechanisms built in to Android and iOS?
Carrier locking prevents this. If someone steals your phone on the train that’s a different problem with a different solution