Just as you aren't supposed to attach the break away cable to the drawbar of a trailer hitch, I was under the impression that for a fifth wheel the cable is to be attached to the truck, and not any part of the hitch.
The theory is that if ANY part of the fifth wheel hitch fails, by attaching the cable to the truck (like one of the tie down loops that is in the truck bed) the entire fifth wheel hitch could be completely torn out of the truck bed but the break away switch would still be activated- however ridiculous that seems.