My guess would be that during launch and reentry the astronauts are wearing pressure suits that include a backpack, are custom fitted and probably provide padding. In orbit theres no need for padding on the seats.
^ Too late for me to edit this, I should have said fully pressurised. The LES, the suit used on shuttle missions up to the late 90s, was partially pressurised.
http://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit