Simple. The seatbelts restrain your body so it can't move. Therefore the seat can't move. The loads go into the frame not the seat. If that's not the case then you need to look at your seatbelt design.
Edit. Steve C you have got it right.
If a car ever gets airbourne and hits the deck the force of your bodyweight is transmitted downwards through the floor, most seat belts only restrain you towards the back and down.
No belts actually take your weight and suspend you!
So your seat does do some work!
You cant truly beleive you can remove the floor section place a jack under the car where your seat was bolted in, rest your seat shell on it, get in and strap your harness on, get somenone to let the jack down and your good to go?
Cheers
Joe T