What’s the identity of the device ? There can be multiple identities based on whether the device is identifying itself to a user, to another device of the same type, or to other devices in the ecosystem that it is a part of (say a component of a car).
Having a unique device id and leveraging it for the services that are built on the device is a design choice. Consider the choices for iPhone and Android. In the iPhone the device id permeates the application layer; the application developer and can target his application for specific devices and must register the device for developing on it. This design choice allows the device to check the applications that are run on it are valid and their associated developer is registered with Apple. It strengthens the associations in the ecosystem of devices, developers, applications and users.
In Android the security certificates were at the JVM layer which allows self-signed certificates. Here the device id is not used as a strong identifier that is known to applications and developers. This is one reason the open system is more prone to malware.
A unique hardware identity is something to look for in IOT designs. Here’s an article from Intel/McAfee discussing EPID an immutable device ID that can be used for identifying and also anonymizing. https://blogs.mcafee.com/business/intels-iot-gateway-enhancements/
Update: On Nov 25, news came of a number of IOT devices using the same HTTPS certificate and SSH keys. See here. Large clusters of devices on the net are exposed on the internet this way.