It's a butt covering exercise designed to minimize the risk of being "the airport" where an incident originates.

Many years ago, NRT was "that airport" (the Air India bombings) and in an effort to avoid losing face in front of the international community in the future, decided to put their own controls on airport security so everyone departing NRT met the same minimum standards.

Frankly, if I could fire every TSA employee and replace them with the staff from Narita, I might actually enjoy going through airport security (at what other airport would a bag search result in an overly apologetic screener insisting they re-fold and pack my clothing even better than they found it).