The back end has to be able to trust the data from the sensor, and the only way to do that is engineer them as part of the same cryptographic trust domain. Othe

The back end has to be able to trust the data from the sensor, and the only way to do that is engineer them as part of the same cryptographic trust domain. Otherwise you could just swap the sensor for a dummy device that presents whatever data you like to the back end. That would completely break the security model. I know a lot of the other phone manufacturers do stuff like this and think it's ok, but Apple knows what it's doing.