Because this is one of those books where the author is either trying to get people mad, just to see their reactions, or they really know what's going on and are the first to say so, and therefore be burned at the stake.
First, this book makes me think and makes me want to look further into Ruby on Rails and continuation servers. They sound interesting.
Second, some of the things Bruce says in this book seem to contradict themselves when it comes to him sounding off at Java's shortcomings.
I think some of his "pluses" for using Ruby is because he hasn't used a dynamic typed language before, and not seeing the trade-offs and other maintenance problems that come with dynamic typing.