It sounds like a very magnanimous thing for Google to do – to build a virtual library of Alexandria, but there is a solid business reason as well.
One of the simplest ways to game Google is to scan out of copyright books, rare ones ideally, boost Pagerank by buying hard links, and serve Adsense against the results. This is commonly done currently, with specialist Dictionaries.
However, Pagerank only really works if you have original content, i.e. stuff that is not already on the web; slapping up a copy of the works of Shakespeare won’t do. If Google scans out of copyright books, and serves them up itself, then attempts to trick Google into handing out Adsense revenue without generating any content will not work.