Hispanics aside, the evidence strongly favors the hypothesis that the police are looking to arrest and convict as many drug dealers as possible, regardless of race. Now what's wrong with that? The answer depends on how you feel about the Drug War.
If, like me, you consider the Drug War a moral outrage, you'll be distressed to learn that the police are maximizing drug convictions. Stopping motorists because you don't like their race is reprehensible, but at least it doesn't retard economic activity. If the police are going to harass a dozen motorists a day, it doesn't much matter whether they target blacks, whites, or a representative sample; 12 harassed motorists are 12 harassed motorists. But it does matter whether they target drug dealers, because that discourages the drug trade and raises the price of drugs.
Of course, if, unlike me, you're a committed Drug Warrior, you'll consider discouraging the drug trade a good thing. So you might think a committed Drug Warrior would applaud a police policy of maximizing drug convictions. Knowles, Persico, and Todd argue otherwise: If you really want to retard drug traffic, you should be maximizing not convictions but deterrence. And to maximize deterrence, you should probably stop more whites, because there are more whites in the population to deter.
Searching mostly blacks can be simultaneously a very good way to make lots of drug arrests and a very poor way to slow down drug traffic. That's because it advertises to whites that they have little to fear from the police, which emboldens more whites to carry drugs. And because there are so many white people around, this effect can be quite large. After all, one-third of whites represents a lot more motorists—and a lot more drugs—than one-third of blacks.
So, whether you're for or against the Drug War, you've got a good reason to emulate the ACLU and call for a more racially balanced stop-and-search policy. There would be fewer arrests (to appeal to the libertarians) and greater deterrence (to appeal to the prohibitionists).
Of course, that's not what's on the mind of the ACLU types: They're concerned about racial fairness for its own sake. I argued earlier that racial fairness doesn't much matter, because a harassed white and a harassed black are equally harassed. But here's a legitimate counterargument: When a minority is targeted, the same individuals get stopped over and over, and perhaps being stopped 10 times is more than 10 times as bad as being stopped once. So I'm willing to count racial fairness as an additional minor plus in favor of the ACLU's position.
The bottom line: The police aren't racists. Instead, they're out to jail as many people as possible, regardless of race and regardless of the actual effect they have on drug traffic. That's not racism, but it's still not very pretty.