OTOH, this appears to still happen if the phone itself is reset to a factory image. It doesn't seem to be that much of a security risk if instead of refusing to work, the phone, after being reset, would renegotiate encryption with the sensor. There's no data to be stolen in that scenario. And there's other mechanisms to prevent a stolen phone from having resale value.
It's still a security risk. You could imaging intercepting new iPhones, replacing the fingerprint sensor with a compromised one containing a backdoor, then reimaging the phones, putting them back in the box, and selling them to your target. After your target loads their sensitive data on to them, you could then retrieve it using the compromised sensor.
I agree this is somewhat contrived and Apple is likely just looking to block third party repairs, but it still is a valid security risk.