Kevin,
My understanding is that you are not allowed to use them to drive to the summit. Makes sense, since that paragraph deals with driving to the trailhead. In other words, you are not allowed to drive up the Auto Road to the parking lot, call it a trailhead, and hike from there to the summit.
I am almost certain that I am correct, but hope that Eric will give us the official answer.
Jim,
I remember reading (maybe in an old WMG) that the auto road can be treacherous (ice and wind) in winter, and that it is not an easy way up the mountain.