Ah -- I might have found the catch with that Kobo Plus deal. It doesn't give you access to their entire catalog, just some titles:
"Not every eBook or audiobook from Kobo is available to read with a subscription. There are over 1.3 million eBooks and over 100,000 audiobooks currently available through Kobo Plus, with more being added all the time.
Any time you see an eBook or audiobook with the option Read with Kobo Plus, it's available to read with your subscription."
A quick browse suggests that most books in their catalog aren't covered by this program.
