Law enforcement officers are public servants. Taxpayers foot the bill for their salaries, equipment, and sometimes their peccadilloes (or at least legal settlements). Police officers belong to public-sector unions, like other public servants. But police unions are not like those other unions, just as police are different from the often faceless bureaucrats who keep state and local governments running.

These differences are starkly illustrated in the grudge match between the New York Police Department and New York Mayor Bill de Blasio.

The fight is national news: Egged on by their unions — and especially Patrolmen's Benevolent Association head Patrick Lynch — NYPD cops have been turning their backs on de Blasio at funerals and other public events since the cold-blooded murder of two uniformed officers, Rafael Ramos and Wenjian Liu. More significantly, they have sharply curtailed their enforcement of the law in the past two weeks.

The cops and their union chiefs accuse de Blasio — with often histrionic rhetoric — of inciting violence against the police by allowing protests over the death of Eric Garner and implicitly accusing police officers of racism. As evidence, they cite de Blasio's campaign condemnations of stop-and-frisk tactics, his comments about warning his black son to be wary of the police, and something about Al Sharpton.

Politically, the standoff has created an interesting but predictable dynamic: Republicans, generally foes of public-sector unions, are mostly siding with the police unions against de Blasio; Democrats, defenders of public-sector unions and other organized labor, are accusing the NYPD and its unions of thuggery and insubordination.

Why is that predictable? As Eleanor Clift noted at The Daily Beast a month ago, before the murders of Ramos and Liu and the resulting ugliness, Republicans have supported police unions (as well as firefighter unions, but none other) since the Nixon administration. Why? "The police are the domestic version of national defense," explained University of California at San Diego political science professor Sam Popkin. "Republicans love them so much," he added, because they "identify very strongly in a time of change and turbulence with the troops that provide order."

Democratic demographer Ruy Teixeira has a more prosaic explanation: "Not only are the police likely to vote Republican but the union is the mechanism to mobilize them... It's about knowing which side your bread is buttered on."

If you want the police union side of the story, you can read Wall Street Journal editorial board member Dorothy Rabinowitz; for a slap-down of the unions, read this New York Times editorial. The New York Daily News stakes out a middle ground, calling on de Blasio to apologize and the police union to "recognize that their wildly heated rhetoric and not-so-veiled calls for a slowdown are imperiling New Yorkers and their members."

But partisan political considerations aren't the only difference between police unions and their public-sector brethren. In the early 20th century, labor organizers were slow to accept police unions into their fold, since police were frequently called on to crush strikes, sometimes violently, Ned Resnikoff explains at Al Jazeera America. The largest U.S. police union, the Fraternal Order of Police, isn't part of the AFL-CIO or its main competitor, and the International Union of Police Associations didn't join the AFL-CIO until 1979.

There's also another big dissimilarity, and it's so obvious it may be overlooked: Police have guns, and we give them broad powers over our freedom. As Damon Linker notes at The Week, the NYPD is essentially "a modestly sized military force deployed on the streets of the city." The New York City Department of Environmental Protection doesn't have that kind of power. The Atlantic's Conor Friedersdorf admonishes conservatives for not seeing this distinction and its practical consequences:

What's unfolding in New York City is, at its core, a public-employee union using overheated rhetoric and emotional appeals to rile public employees into insubordination. The implied threat to the city's elected leadership and electorate is clear: Cede leverage to the police in the course of negotiating labor agreements or risk an armed, organized army rebelling against civilian control. Such tactics would infuriate the right if deployed by any bureaucracy save law enforcement opposing a left-of-center mayor. [The Atlantic]

If that sounds a bit hyperbolical, David Firestone at Quartz reminds us of the New York Patrolmen's Benevolent Association's "long history of bullying any critic of the police." In fact, he writes, the city's "police unions have attacked and often slandered every recent mayor, even those who prided themselves on being crime fighters."

De Blasio is getting off easy (so far) compared with David Dinkins, whose tenure as mayor ended soon after 10,000 union cops swarmed City Hall to protest the proposed strengthening of a civilian oversight board. The unions even turned on Rudy Giuliani, who participated in that 1992 protest against Dinkins, after he was elected mayor and refused to give the NYPD a generous contract.

Maybe the most concrete recent example of how police unions differ from other public-sector unions comes from Wisconsin, though. In 2011, when Gov. Scott Walker (R) pushed through a radical neutering of the bargaining rights of public-sector unions, he mostly exempted police and firefighter unions.

When Democrats accused him of making a pre-election deal with police unions — some, but not all, backed him in his gubernatorial bids — Walker called the charge "utterly ridiculous." He let the cops keep their collective-bargaining rights because he thought public employees might walk off the job to protest the changes, and with the police and firefighters "that's not an area to mess around with."

It's not favoritism, see, it's fear. Take that, bureaucrats.