They think that it is a reasonable policy for newspapers not to mention the race of a criminal, for example, because to do so might prejudice people against Blacks. You might argue that if a Black rapist or a Black mugger is on the loose, our people should know about it, so that they can protect themselves. They should know what he looks like. Yet, the liberal will argue that since not all Blacks are rapists or muggers, it's bad to mention the race of some who are, because that will cause our people to be wary of Blacks generally. And of course that's true.
People do generalize.
People do stereotype. That's why we're still on this earth. It's a survival trait. Our ancestors a million years ago saw what happened when one of their people got bitten by a poisonous snake, and they began avoiding snakes generally, even though many snakes aren't poisonous. Better to be safe than sorry, they thought, even though our bad opinion of snakes generally might not be justified.