This is the way it has been for a while it's like that for every flash. It has deal with compensation, not power (ie, use 1 more stop of light than it calculated you will need). I guess Nikon has decided that if you are using iTTL, that you only need at most one additional stop of light.
Most likely, this is due to the fact that most lights are located fairly far away from the subject (at least 10-15 ft or more), and so adding more than one stop of compensation will likely take the flash past it's maximum value. People might get confused when adding +2 or +3 doesn't make their lights brighter than +1.5, and would end up complaining. That's my guess anyway.
If you want more power, I am supposing that it makes more sense for you to control the power manually. I don't use iTTL, so I'm not really in a good position to comment on that.