I think I found the answer, at the Google Book Search Help Center:

Why is the Download button only available on some books?

Only books in the public domain -- books whose copyrights have expired -- have the download feature available. For users in the United States, this typically means books published before 1923. For users outside the U.S., we make determinations based on appropriate local laws. As with all of our decisions related to Google Book Search content, we're conservative in our reading of both copyright law and the known facts surrounding a particular book....

So it appears that Google knows what country you're in, and provides different content accordingly. Who woulda thunk it?