The owner of a phone should be able to back up all of the data on it, regardless of the wishes of anyone who doesn't own said phone.

I don't disagree, but I vote with my wallet. If the owner of the phone doesn't agree with that, they could stop using the phone/apps. Similarly the biometric data also doesn't get backed up or transferred over to a new phone. That's a product decision and I'm just explaining why things are done that way on iOS.