If I recall it was head impact requirements to the windshield header when tested with an un-belted dummy.
Ford even proposed a seat-belt-interlock..((.remember the olden days when those were mandatory for a year or two before public outcry killed it ?)). The US Govt said 'NO' to the proposal. It would have meant an all new roof, header , seat, package design and test program to do otherwise..and we just could not afford it..
But Please, I may not have all the details correct and someone like Jamal or Ressler or FredGoodnow really needs to be the final input on this..
AJB (Andy)