Unlimited usage != unlimited speed.
Feel free to use 2TB on your phone, not from another device.
Why stop usage altogether after 7GB when you can use as much as you want, albeit at a lower speed?
Not all usage, only usage originating from their "hot spot" feature on the phone. They can track the data throughput from that service, then cut off the hot spot feature so you can't tether, but keep regular data on the phone as normal.