Media & stereotyping.
That's not to say such portrayals are morally or ethically justified. The statistical truths are often used to support sweeping statements.
MadCow hit the nail on the head. Racism isn't much an overt issue anymore, but it still exists. If nothing else, I can attest to the fact that it does exist in my specific locale, my school, and my demographic.
I attend a private 4 year college, and live in southwestern Connecticut, a relatively wealthy region. The people here and the people I've met who attend my school don't treat minorities any differently. They do, however, think of them differently.
My understanding is their treatment of minorities is attributed to the mannerisms in which they were instructed as youths, whereas their preconceptions have been largely formed by stereotypes propagated by the media. FWIW, I behave in the same way. I don't believe I'm racist (hell I'm a minority), but I do recognize that I should not think the way I do. But as long as that doesn't affect how I treat people, it's a flaw I'm willing to live with.