The Justice Department report released Wednesday on police practices in Ferguson, Mo., makes agonizing reading — and not only because of its careful documentation of both patterns and specific instances of racial discrimination by law enforcement in that troubled city. An important subtext is that many of the problems stem from a decision made by politicians to treat the police department as a cash cow.

The Justice Department's conclusion on the matter is right at the top of the second page of the report: "Ferguson's law enforcement practices are shaped by the City's focus on revenue rather than by public safety needs." This emphasis, in turn, has contributed to the city's "pattern of unconstitutional policing."

In 2010, the city's finance director wrote to the chief of police to warn of a shortfall "unless ticket writing ramps up significantly before the end of the year." Three years later, the finance director asked the chief to try to increase revenue to cover rising court fees.

Evidently, these pressures had the desired results: "Ferguson police officers from all ranks told us that revenue generation is stressed heavily within the police department, and that the message comes from City leadership."

And then there is this: "Patrol assignments and schedules are geared toward aggressive enforcement of Ferguson's municipal code, with insufficient thought given to whether enforcement strategies promote public safety or unnecessarily undermine community trust and cooperation. Officer evaluations and promotions depend to an inordinate degree on 'productivity,' meaning the number of citations issued."

As the Justice Department report goes on to detail, the burden of this form of policing has fallen most heavily on Ferguson's black community. But even were the costs more fairly distributed, the central problem would remain: The city is using its police force not as a tool to enforce the law but as a tool to raise revenue. Once that decision is made, corruption and overreach are inevitable.

In this, Ferguson is hardly alone. It's awfully late in the day to pretend that many police departments don't see themselves as in the business of raising revenue — not when professional publications feature advice on how to go about it. A 2009 article in Car and Driver quoted the head of the Police Officers Association of Michigan: "When elected officials say, 'We need more money,' they can't look to the department of public works to raise revenues, so where do they find it? Police departments."

Sometimes the money comes from speed traps. Sometimes it comes from tightened enforcement of forgotten ordinances. Whatever the case, the result is to distract the police from their central mission by focusing their attention on offenses that result in fines. And the fault doesn't lie with the cops; it lies with the politicians who set priorities.

Traffic cameras at intersections have been a particular bone of contention. Although some studies suggest that the cameras do indeed reduce the rate of accidents and injuries, both motorists and politicians seem to think that the primary function is to raise money. Software and hardware problems often lead to incorrect or uncollectable citations. And in places where public opposition has led to the abandonment of the cameras, municipalities have had to scramble to make up the lost income. Washington police conceded this year that a drop in ticket revenue came not because motorists were now obeying the laws but because so many of the city's traffic cameras were broken.

Perhaps the best example of the corruption spawned when law enforcement becomes a revenue source is the estimated $3 billion earned by state and local law enforcement through asset forfeitures under the Justice Department's Equitable Sharing Program, which was halted by Attorney General Eric Holder in January. Of course, the states can continue to operate under their own civil forfeiture rules, which generally allow the police under certain circumstances to seize property without warrants or indictments, and then place significant legal obstacles in the path of those who try to get their cash or belongings back.

It's not clear that asset forfeiture was ever a good idea, but it has certainly become a bad one. Last year, John Yoder and Brad Cates, two of the architects of the programs when they worked for the Justice Department, called for their abolition: "Asset forfeiture was conceived as a way to cut into the profit motive that fueled rampant drug trafficking by cartels and other criminal enterprises, in order to fight the social evils of drug dealing and abuse. Over time, however, the tactic has turned into an evil itself, with the corruption it engendered among government and law enforcement coming to clearly outweigh any benefits."

But the potential for corruption should have been evident from the start. Government is never shy about stretching its powers as far as they'll go. That's why limits are so important. Raising revenue is not a proper function of law enforcement, and politicians shouldn't be pressing cops to fill municipal coffers.