It isn't in favor of AT&T. At all. Having platform lock in means after they sell you a device they can price gouge you indefinitely and since you probably bought the device outright since you aren't on a 2 year contract and thus have the option to change carriers, you probably don't want to just ditch it immediately.
Platform lock in is good for device manufacturers, carriers, and business, but bad for consumer freedom, choice, and market competitiveness. And there is no multi-billion dollar lobby for the the latter.