Interesting Google Book Search Happening

Something interesting happened to me on my way to a Google Book Search: I was prompted for my login.

I recently picked up my copy of the Wallace Stevens: The Collected Poems and read the poem "Winter Bells." Wanting to know more, I started out on the main Google page, but didn't come up with what I was looking for, so I figured I'd give Google Book Search a try.

One book had restricted content, but when I clicked on another a note came up asking me to log in. I did without even thinking, but a second later I wondered why. I don't need to log in to read any other searches I find from Google.

I'm not sure if this was related to the specific book I selected, whether it's something the publisher requested, or if it's standard practice. I've looked through the site but can't find any relevant information. If anyone has more information I'd be interested in finding out.

Added:
A few minutes after writing the above I answered my own question when I ended up in a FAQ that I didn't see before:

Why do I have to log in to see certain pages?

Because many of the books in Google Book Search are still under copyright, we limit the amount of a book that a user can see. In order to enforce these limits, we make some pages available only after you log in to an existing Google Account (such as a Gmail account) or create a new one. If you prefer not to log in but still want to see a few pages, click the "view an unrestricted page" link. Remember, the aim of Google Book Search is to help you discover books, not read them cover to cover, so you may not be able to see every page you're interested in.

Interesting. I don't know if this was part of the original plan or something added after the controversy.