Most sushi places use frozen tuna because they do not have access to steady supply of fresh, unfrozen tuna.
I know of a couple of high end sushi restaurants that only use the fresh stuff, but they also charge accordingly.
Your local sushi place can't afford to use the best stuff since they probably do not have customers who appreciate them and are willing to pay for them.
Sure, freezing eleminates any possible parasites, but to me, that defeats the whole purpose of catching tuna. Besides the fun factor, I truly enjoy eathing fresh, never frozen tuna, and I just cannot get it anywhere. So I catch my own.
Regarding parasites, what I noticed is that there are worms in the intestines of tuna. If the tuna is not immediately bled and gutted, the worms are eventually going to make their way to the muscle. That's why I love going on Voyager, since they take the best care of my catch. In 10 mins, fish is bled and gutted, and dunked in ice bath.
