And they lend them out. Which is considered fair use.
Just like full-text search, now.
As was said by a sibling comment to yours, people do this all the time in bookstores too.
Corollary: Google shows a snippet of text from a website under their link. If what I wanted to find out is contained in that snippet, I won't necessarily click the link. Same thing, but instead of a pointer to a website (which, in this example, just lost a pageview), it's a pointer to an book that you can purchase.
More funnels to conversion mean more books purchased.