[Nightflyer]

(I think) There is a difference between
1) owning or borrowing an ebook, and
2) having a specific piece of hardware in this case called a Kobo to read it on.

The books are still available even if a specific Kobo ereader is not accessible to it.

Kobo sells an ebook, and sells a piece of hardware to read it on.

The ebook is still available, even if that specific Kobo ereader piece of hardware runs into problems. The ebook can still be accessed through other methods.

Logging in gives a specific Kobo device authorization to access purchased ebooks related to a specific account, and separate authorization to put those books on that specific ereader.

There are apparently ways to use Kobo ereader without registering with Kobo. So can still use it outside of Kobo ecosystem for things like side loaded ebooks.