Gah! Yes, that bothers me so much too! I mean, it's good that some people try to cut out at least some animals from their diets, but they shouldn't be calling themselves vegetarians. Usually I've found that the people who do that are the same ones who try really hard to appear "alternative." It's an image thing, especially for teenagers.
Unfortunately I'm usually too shy to say anything to people like that, or I feel like I'm butting into a conversation by saying anything. Plus, I don't really like to divulge that I'm a vegetarian when I don't have to, since it always ends with the same stupid questions. I know, I should get over that shyness thing.