Formaldehyde
Both Fair And Balanced
EDIT: Ah. You are specifically referring to only uncooked fish. Yeah. All fish other than tuna must be frozen first due to health concerns. None of the rest is usually local fish anyway, so you wouldn't want it if it had not been frozen for the long journey.Sushi itself ofc is not illegal. IIRC though, its illegal to sell raw fish that hasnt been frozen before in the US (or maybe its just ny i am not quite sure). freezing the meat really degrades the quality of sushi, but AFAIK its the law and most restuarants comply with the law, unless you know some chefs well and he is hooking you up with some not so legal stuff.
There is also a matter of season. Even tuna is frozen out-of-season.