It's not impossible, it's just expensive to import and difficult to store. Higher end sushi places would usually have it. Now there are at least two farms in the US producing it that I know of, one in Washington state, and another in Half Moon Bay, near San Francisco, and there's even some retail distribution now in SF.

That is very exciting! My old karate instructor owned a sushi restaurant and I got the strong impression that the mob looked poorly on attempts to export. He may have been exaggerating.