I am in the United States, and I have walked into multiple vegetarian restaurants in the last year that have had fish in their menu. When asked they simply say that they consider it a "sea-vegetable". I don't find that to make any sense, but this was the same case when I visited Malaysia and few restaurants in Singapore as well. From friends and family, I have also heard this being the case in some European countries as well.
Why do restaurants do this?