Facebook COO Sheryl Sandberg’s organization, LeanIn.org,  came under fire Friday for advertising an unpaid internship opportunity. The “must work for free” line in the job description didn’t sit well with a handful of lady bloggers, who decried the unpaid nature of the job as incompatible with LeanIn’s mission of “advocating for women’s rights.” The organization tried to cover its tracks, now seeking “volunteers” instead of “interns.”

But LeanIn.org did nothing wrong. Unpaid internships offer many people an opportunity to enter or re-enter the workforce that they otherwise wouldn’t have. This is especially true for women, who according to InternBridge.com, hold 77 percent of unpaid internships.

Critics of unpaid internships often focus on the “free labor” for firms and disregard the (non-monetary) rewards for workers.  Internships often function as a trial period for workers to prove themselves before transitioning into a paid position.  Even if this doesn’t happen immediately or within the same firm, interns gain valuable work experience, new skills and new references for their resume.

Similarly, while most people assume that the cost to the firm of offering an unpaid internship is zero, this does not account for the time and resources spent training or managing interns. But both parties – employers and interns – signal that the benefits outweigh the costs when they decide to participate in this exchange of “free labor.”

Some people, even like conservative Charles Murray, have suggested that unpaid internships contribute to an opportunity divide among socioeconomic classes. It seems plausible that unpaid opportunities are more likely to benefit wealthy youth whose parents can fund their living expenses for a summer or semester internship.

But the solution to this is not to do away with unpaid opportunities.  Too often when we seek to “level the playing field,” we level it with a wrecking ball, rather than by filling in holes.

Many colleges now offer stipend or scholarship programs to students who want to spend the summer interning.  There are also foundations dedicated to connecting prospective interns with grant money or other awards. These programs and other private efforts are a better solution than outlawing unpaid internships.

In fact, requiring firms to pay interns would come with negative consequences.

In a difficult economy, many small firms or nonprofits do not have the margins to take risks on inexperienced workers. And given America’s current youth employment crisis, we should not be closing one of the few remaining doors to professional work.

When government mandates that employers provide a certain (minimum) wage or other benefits, like health insurance or maternity leave, we see decreased employment levels as firms can afford to employ fewer workers. Requiring firms to pay interns would have the same effect: It would raise the cost for firms who offer internships, and fewer internship opportunities would be the result.

If you don’t believe that an unpaid internship is the best opportunity available to you, you may be right.  You may be able to find paid work.  But unpaid internships should remain an option for workers who choose to take advantage.

As someone whose career began with an unpaid internship (at a nonprofit women’s organization, no less!) I know firsthand that a work-for-free opportunity can turn into a job. I had a scholarship that (barely) paid for a 9’-by-9’ room in Washington, DC, and my food, which was mostly comprised of groceries that I cooked in a shared hall-kitchen.

Three years later, I’m happy to say I’ve been employed full-time with the same organization that benefited from my free labor in summer 2009. But really, who benefited?

Clearly, unpaid internships exist because both parties, the employers and the workers, decide that they will benefit. There is no shame in this. Anyone who decides to intern – wait, “volunteer” – at LeanIn.org will have make that same calculation.