Interesting question. I kind of wondered about that when I read it in the manual. I can't think of a logical reason for it - unless there is some system that keeps churning away even when the power is off (vehicle stability, or some such, using speed sensors at individual wheels.)
I recently pondered this, as my wife and I were going to be traveling to a location a few hours away, at different times, and her AWD Pilot is rated to tow 4,500 pounds. My thought was to rent a tow-dolly and pull the Clarity home with it. It turned out that we were able to arrange to drive back separately, too.