I also thought that was odd, at first. But now that I think about it, the rear speakers in my car sit right behind the passenger's head. It has become quite often that I have passengers which don't always appreciate the excessive volume I enjoy. Thus, having a good deal less power for the rears, where the speakers are extremely close ot the listener's ears, does actually make sense to me.

And of course some might argue that this would help to significantly pull the sound stage toward the front of the car, which is where it should be for stereo audio anyway.