You are correct bone but only for some seats. The solid framed seats should have a seat back brace that attaches the back of the seat to the roll cage/bar. Other seats are designed to flex slightly (non-adjustable seats so they don't snap back onto the roll bar) to actually absorb some of the impact of you coming to a dead stop...
Much like factory crumple zones. since those were taken out when you put a cage in, it's nice if the seat can do some of that work
