That's right! In US all fish served raw (like for sushi) has to be previously frozen in order to ensure that all parasites are dead. The only exception is tuna. You can serve raw tuna that wasn't frozen because it's not prone to parasites. But that's sushi. When it comes to fish used for cooking, I've never seen previously frozen fin fish in upscale restaurants. Shellfish is another story. It's almost impossible to get fresh shrimp in US anymore, so most of the shrimp is previously frozen. Of course, this comes from my very limited experience on working in a restaurant in Boston, and I am not sure if this would be the case everywhere. But considering that half of the fish sold fresh in Boston is from the west coast, I don't see why they can't ship it to inland states as easily as from coast to coast.
I'm not sure about the restaurants' avoidance of frozen fish. There have been articles in the past couple of years about how much of the tuna and salmon served as sushi and sashimi is frozen to a very low temp, in part to kill parasites or bacteria.
After reading so much about how frozen fish can be just as good as fresh (even Mark Bittman says that), I headed off to Trader Joe's to get some. I bought a frozen halibut steak that was priced at half the price I pay for fresh. I must admit -- I really wanted it to be good! If it tasted as good as fresh I would have an answer for all my students who complain about fish being so expensive. Unfortunately, it lost a lot of water during defrosting even though I did it slowly in the fridge, and didn't taste nearly as juicy as fresh. Of course, I gave it a tough test -- steamed with ginger and scallions. Saucing it would be a nice cover up. Thinking I defronted the fish incorrectly, I tried buying some already defrosted wild salmon from Whole Foods. Same results.
Here is one thing to keep in mind. Most of the time, frozen fish is cheaper than fresh. If the frozen fish we buy in the stores was the same as sushi fish, I doubt it would be that cheap.
Frozen fish still puzzles me. I'll ask local fishmongers about it and will report back to you guys. Meanwhile if you have tips on how to buy and defrost frozen fish to make it taste as good as fresh, please drop me a line. I am very curious.