Why is milk - produced from an animal - considered vegetarian?

Well, milk is a dairy product such as cheese, just because it comes from an animal doesn't mean that it has any animal content in it, so it's okay for vegetarians to have milk, cheese and yogurt. However some strict vegetarians i.e vegans would avoid dairy products because male calves produced in the dairy industry are an unwanted product of the dairy farming system and are more often than not killled shortly after birth.