It's a safety modification.

See... if you and another car directly behind you are going real fast down the straight and your engine suddenly goes south, then the other car will hit you in the rear. This will accelerate your car violently forward (your body will move relatively backwards) and the mandated seat support brace will puncture your seat and poke you in the spine. If you have accidentally mounted your seat support brace so it won't poke a hole in the seat then your entire head/neck will whip forward and you could get a migratory sprain/strain whiplash condition. A spine poke or whiplash is not a good thing and therefore this engine modification is definitely done only in the name of safety.