It's just another example of stereotypes in action. Each of us is stock-full of such preconceptions. We're a label-happy society. And, of course, there are the labels borne of hatred. We often label ourselves.
Labels definitely have their purposes.
While the term has developed a negative connotation, stereotypes are neither good nor bad in themselves but merely designate a natural way in which each of us organizes the massive influx of information we receive each day. We come across a situation that is similar to one we've seen before and instead of having to work out every detail of how to deal with it, we rely on our store house of accumulated knowledge. The less important the task, the less we have to consciously think about it, leaving us time to think about more important matters. Stereotypes help us organize the world, make our lives easier and often prove accurate, but, with overuse, reduce our ability to analyze the world around us and have the potential to become stifling prejudices. They injure us and those we apply them to when we fail to take the time to see people as individuals.