I'm really not sure (again, I've never seen bonito in a restaurant on the East Coast). It could be that Pacific bonito have some other parasitic critters that searing takes care of, but I'm not familiar with anything of the kind in the Atlantic species. Another possibility is the way Pacific bonito are harvested. Commercial boats take them as by-catch on the way to off shore fishing grounds. Even well iced, they probably pick up some nasties at the bottom of the hold.