… ‘cuz they’re doing Mayor Bloomberg’s job as well as their own (and the mayor gets paid $1 a year).

Some background: Michael Mulgrew and Lillian Roberts head up New York City’s United Federation of Teachers and the city’s largest civilian union, DC-37, respectively. The two unions’ members make up the bulk of the New York City workforce.

During this year’s budget season, ended last Friday, Bloomberg left it entirely up to Mulgrew and Roberts to decide how many employees New York City needs to deliver adequate public services to taxpayers and citizens.

How? Bloomberg decided that union generosity — i.e. cost “givebacks” — would determine whether the city experienced teacher layoffs as well as civilian workforce cuts.

In the end, to avoid 4,100 layoffs, Mulgrew decided to go ahead with a superficial concession, allowing unassigned teachers to work more as substitutes (as the Post says, shouldn’t we do this anyway?). Roberts held firm, deciding to let Bloomberg go ahead with 1,000 layoffs.

But it is not union leaders’ job to decide how many workers New York needs. If the city needs those 4,100 teachers, it’s the mayor’s job to find a way to pay for them. Same thing with the civilian workforce.

On the other hand, if New Yorkers don’t need this many public employees, it’s the mayor’s job to break the bad news — and not to treat layoffs as a punishment for bad behavior.

The mayor’s approach comes straight from “That 70s Show.” In the Seventies, New York leaders often relied on the unions’ cooperative spirit to avert dire consequences for the city.

But much of the rest of the country is moving on. In New Jersey, Gov. Christie and the Democratic legislature moved last week to assert more management control of the workforce. They took away workers’ right to bargain for healthcare, so that management can better control worker costs. Wisconsin has done the same thing.

Gov. Cuomo, like Bloomberg, went the cooperation route — wringing three years’ worth of wage freezes from the state’s largest workforce last week to avoid layoffs. As E.J. notes, this sacrifice and others parts of the deal are “mostly temporary.”

We’ll see, too, if the state workforce even ratifies the deal, as Connecticut workers just rejected a similar compromise. (Most workers in the Nutmeg State actually voted for the deal, but a quirk in inter-union rules there means that the majority doesn’t prevail.)

On the bright side, maybe voters should thank Bloomberg for adding more results to the natural experiment that’s playing out all over the country this year.

If you wait for unions to act charitably, you’ll keep waiting.