Why should they have such restrictive control over the device in the first place? After a successful contract completion, the device should be the end-users 10

Why should they have such restrictive control over the device in the first place? After a successful contract completion, the device should be the end-users 100%, but it seems like the telecom's are futilely fighting to control the balance of power by "dumbing down" the absolute potential of the devices they release. This is truly scary.