[niemeyer]

I work for Canonical on snapd, so can provide some background here.

You're probably describing Ubuntu Core instead of classic Ubuntu. The UX there is oriented for devices, and it was cooked to avoid default passwords in an environment in which the device often will have no display. So once you boot, the device is in a running state, and the brand (manufacturer) that cooked the image has the choice of allowing individuals to login or not. In addition to a store account, the brand can also offer a "system-user" assertion, that is a signed document that you can present devices to get a system user in. That assertion may detail remote login, SSH keys, and also a hashed password for independent logins. That only works once on the device, though, for obvious reasons.

For generic Ubuntu Core devices the "brand" is Canonical, and for those devices you can get an assertion signed and with it log into any number of devices you want. That procedure may be done over USB storage, for example. Just insert a USB key into the device and your user credentials will be setup, even if it's completely offline. Again, that only works once on the device. If you lose the keys the device will need to be factory-reset.

[dserodio]

Great explanation, thanks