Why should you buy it when it's free to read online?
Support the authors
Writing a book isn't as vastly lucrative as you might suspect, but every time somebody buys a copy we do get a small financial reward. Selling more copies means we might be able to write more books in the future, or put more effort into working on Mason itself.
Support making the book freely available
O'Reilly has also generously agreed to allow us to make the content of the book available for reading online, at no cost. But if this means nobody's buying the hard copies, you won't likely see such things from them in the future, as they'll either go out of business or stop making books freely available.
Support Mason in the marketplace
Since this is the first book to be published on Mason, its success (or failure) will no doubt influence O'Reilly (and other publishers, for that matter) in deciding whether or not more books on Mason are published.
You get an actual book
Building complicated web sites can be, erm, complicated, and the last thing you need is another window open on your screen when you're trying to work. O'Reilly's books are made of real dead trees, and they're much easier to cuddle up with beside the fireplace than a computer is.