Weight may not be the issue...think more towards SIZE. What happens when you try to over stuff a canolli? It eventully breaks open.
Consider a very large human who stuffed themselves into a seat and had their friend SMASH the shoulder restraint down to the first lock...
With the way FJ twists turns and flips, the should restraints could shift a millimeter, literally a millimeter, and fault the ride because someone's restraint is loose.