PPBM is not required on any flight in the US. What is required is that all bags be screened for explosives. However, that goal was not able to be met, so in the interim the FAA/TSA is allowing PPBM in lieu of screening until all the facilities and persons are in place to screen every bag. Any airline that screens every bag loaded does not have to implement PPBM (which is occurring by some airlines at some airports).