I assume that the decision regarding F/C/Y was a NWA decision and not a TSA one. In the past, did NW have metal knives in Y? I would guess that NW will continue to have all plastic in Y, not metal fork and a plastic knife as was the case before on most airlines, except AA and maybe a few others. I doubt even the TSA would say that only F/C can have metal knifes because of safety reasons. It is probably economic reasons for NW.