You're forgetting that the back seat is elevated. In fact, the front seats (other than the pilot car) are the ones that have obstructed view and it has nothing to do with the headrests.
Incidentally, I suspect that the headrest has everything to do with the 45-degree lift hill. At least Millennium Force has a logical reason for having headrests, unlike at least two other Cedar Point coasters I can name.....
--Dave Althoff, Jr.