You might start reading a book on your computer at home in the morning, and you might leave that open as you rush off to work. You might open a second copy on your e-reader as you go to work, and so on.

Therefore there are natural use cases where you have the same Google account logged into Google ebooks on multiple devices at the same time.

Therefore Google must allow for those use cases.

Those use cases can be mimicked by friends sharing a Google account.

Google could check for obvious misuse. For example, they could look for two devices separated by a large distance and using the same Google account with Google ebooks at the same time. I know they already do something like that with Gmail (for a different reason).