Just got a reply back from Subaru of America's website. Here is their explanation:
"The front seats of the 2006 Subaru models are equipped with active head restraints. They automatically tilt forward slightly in the event the vehicle is struck from the rear, decreasing the amount of rearward head movement and thus reducing the risk of whiplash. For maximum effectiveness, the head restraint should be adjusted so the center of the head restraint is closest to the top of the occupant's ears."
Of course, they do not recommend mods of any type to the head restraint.
For now, we just have to deal with it.