Why does that even matter? Currently, if you have a locked phone and switch carriers, you have to buy an entirely new phone anyways.
At least this way, a user can pay once, and then hop around carriers depending on what's cheap.
Also there's no shot that locking users to phones costs that much because the unlocked version of a phone is only like 15-20% more expensive. Since when did you ever get a 70% discount on the MSRP of a phone for buying it locked??? They're straight ass lying lmao