My guess is a sensor requires a signed connection, much like how HDCP works. Seems a bit over-engineered if Apple is building protected sensors, but would also love to hear from someone in the know.
My guess would be that they shipped with an updated firmware version to the one programmed in the factory. It should have upgraded the first time the device is powered on but because of a missing security signature or maybe key to verify a signature, the firmware update wasn't applied. This is still just a guess but signed firmware is becoming much more common.
