AirTran requires as many customers to check in at the kiosk or online as possible and audits stations on kiosk usage, so the agents at PHF probably preprinted the boarding passes to meet their kiosk goal. This probably saves time for most passengers as I've found AirTran's kiosks to be very slow, but in your case (wanting to change your seat, presumably no bags to check so no need to see an agent) it is problematic.

RIC preprinted my boarding pass from the kiosk once, but the flight had only about 15 passengers on it.