I'd have to disagree - it's a huge deal. There are two reasons this happens, either the buyer doing the sourcing fails to thoroughly investigate his/her sources or (which I believe is the majority case) it's outright fraud - the substitution of a much cheaper fish/shark, no matter how tasty, for a more expensive one the consumer thinks they are getting, and for which they are paying top dollar. Example, some cheap whitefish (for example, Alaskan Sablefish) substituted for Chilean Sea Bass which I've seen for upwards of $30/lb. at the store, and double or triple that at a restaurant.
It's no different than telling a customer they're getting Michelins when in fact they're getting Hankooks (but paying for Michelins!) Hankooks might be a pretty good tire, but they sell for much less than Michelins and the tire buyer should know what they're getting.