Well, no we don't have them common here, at least in my area of Canada. As you say, when they are available they are pricey.
As they are not that common to me is the reason I looked them up on British Websites to see the differences, hence my answer.
Often here, they mix the names of Rock lobsters with giant prawns.
But getting back to your original post was if shrimps were standard in a Korai that you had ordered with King Prawns.
Perhaps they should have described their dish as a Korai of King Prawns & Shrimp. With the King Prawns being listed first then they should be the foremost ingredient. But it is still their choice - to add whatever else they choose. They probably didn't list the vegetables?
Was it at least tasty? Or did the feeling of being ripped off spoil your appetite? You should tell them that you are unhappy. Most restaurants don't mind helpful suggestions/criticism tactfully done.