I have no problem with sharing my books online or with others sharing books. My view is that the more that is shared about the game the better! Seeing new strategies and concepts is fascinating and only helps the community build better books. The constructive criticism for tweaking a book can only be done by posting books in the first place.
I get that others can see the books and copy ideas but it's no different than watching a match online or spectating a tournament game and seeing all of the cards that are played. You get the overall idea of the book and what cards are used for the strategy.
That being said, I am not a advocate of "net decking" where you copy every strategy and card that someone else played for your books. If you are a mage wars beginner it seems reasonable to do that to get a grasp on what cards work with certain strategies. But, after that, I believe that you should be of the mindset "This was a cool idea, how can I make this my own using my new knowledge, play style, strategies, etc."?