A) This isn't what OP was asking about. They're pretty clearly asking about stolen devices from a consumer, not consumers stealing devices from carriers.
B) Your take is complicated by the fact that there actually is a secondary market for locked phones [0], so this isn't just about people rent-to-owning a phone with an explicit installment plan.
My point is that carrier locking is about managing credit risk and fraud, not an evil plot to trap customers, and not a mechanism for discouraging street theft.
It’s only complicated if people conflate issues or fail to understand the mechanics of carrier locking. You can just call up a carrier and ask to have the phone unlocked and they’ll oblige if its paid off. Sometimes people confuse carrier locking with imei blacklists, which is for stolen handsets. Sometimes people confuse phones with modems or firmware that only work with specific carriers as “carrier locked” but again, that’s not the same thing
B) Your take is complicated by the fact that there actually is a secondary market for locked phones [0], so this isn't just about people rent-to-owning a phone with an explicit installment plan.
[0] https://www.ebay.com/itm/186656753206