Let us remember that we live in a world of scarcity, that economizing on information can be efficient, and that sometimes the reason stereotypes exist is because, well, they're true.

I wonder what the difference between "economizing on information" and ignorance is.

I think the author meant that sometimes the need to make quick decisions outweighs the requirement that they are always accurate. I'd rather discriminate a dangerous looking person by stereotyping them, than risk becoming a victim because of my political-correctness. Gut-feeling is not always wrong, you know. And I can always get to know that person later, in more favourable and safe circumstances.

I think you read the author charitably. Yes his examples are of individuals making snap decisions with the available information, but he using those examples to argue for government and policy decisions--the subtext is racial profiling by government agencies and in hiring for jobs. Crossing the street to avoid "dangerous looking" men and actively persecuting those men simply because they "look dangerous" are two very different things.