McDonald’s is now responsible for labor law violations committed in its restaurants — even if the store is owned by a franchisee.

That’s the essence of a finding this week by the general counsel of the National Labor Relations Board, which stems from strikes by fast food workers for higher wages, and allegations of firings and other illegal retaliation. If the finding is upheld, it will be a major victory for millions of workers in restaurants and other franchised businesses. Workers could hold accountable the multibillion-dollar companies that use pricing, leasing, scheduling and other non-negotiable rules to effectively control the employment practices of their franchisees.

Business groups are crying foul. In a Roll Call opinion column, National Restaurant Association president Dawn Sweeney said the NLRB finding “defies precedent and common sense” and predicts “dire consequences to franchisees, franchise employees and the economy as a whole.”

Actually, the NLRB’s top attorney is simply recognizing current economic reality. U.S. workplaces are changing, and it’s important that labor law change along with them. The idea of a lifelong career at a single employer, with stable wages and working conditions, is about as relevant to the lives of most workers as a turntable is to a teenager.

Changing jobs on a regular basis is a fact of life — so is dealing with multiple companies in a single workplace. The International Franchise Association, for example, reports that 9 million U.S. workers are employed in franchised businesses. Another 17 million work as temps, contract or freelance workers. Taken together, that’s nearly 20 percent of the U.S. workforce who get paychecks from one company, but work under rules set by another outfit they may never get to see or talk to.

All too often, both companies try to duck responsibility if anything goes wrong. It’s a particular problem when it comes to an employer’s obligation to provide safe working conditions. Fast food workers, moving hot items at a rapid pace in confined surroundings, are exposed to numerous hazards. Temporary and contract workers are frequently assigned dirty, hazardous jobs that no one else wants, like cleaning out boilers and waste tanks.

According to “Temporary Work, Lasting Harm,” an investigation by the non-profit journalism site ProPublica, temp workers suffer higher rates of workplace injuries than full-time employees. That’s why my organization, the National Council for Occupational Safety and Health, recently joined an amicus brief in support of temps at a recycling plant in Milpitas, Calif.

When Milpitas workers filed a union organizing petition last year, they asked the NLRB to recognize both the company that hires them — Leadpoint Business Services — and the company they are assigned to — Browning Ferris Industries — as joint employers. Seeking representation by the Teamsters, the workers argued that since two companies share control over the work environment, both should come to the bargaining table.

The Milpitas case, like the one involving McDonald’s, is being closely followed. It’s another opportunity for the NLRB to modernize labor law so it matches the actual working conditions of a huge slice of the U.S. workforce.

The law needs updating, as ProPublica discovered, because many companies use confusion about who’s really in charge to evade laws requiring safety in the workplace. Temp workers are paying a terrible price. A few examples:

• “We don’t train temps:” That’s the answer given to U.S. Occupational Safety and Health Administration inspector after Simon Martinez, a 39-year-old contract employee, was crushed to death by three 800-pound bales of cardboard at a Sonoco Recycling plant in Raleigh, N.C. It was the second death at the facility in less than two months.

• “Not responsible:” 24-year-old Travis Kidd, a temp at a company called Workforce Staffing, was killed in 2011 when he was run over by a trash compactor at a Cleveland County, N.C., landfill. “Landfill management,” OSHA reported, “felt they were not responsible to require or provide Mr. Kidd with the same PPE [personal protective equipment] because they considered him a temporary employee and not their employee.”

• A temp’s temperature — 106.9 degrees: On May 29, 2012, Mark Jefferson, a former high school basketball star, went looking for work at Labor Ready in Trenton, N.J. He was assigned to a truck operated by Waste Management. After a nine-hour shift in 90-degree heat, he collapsed and never recovered; his internal temperature was recorded at 106.9 degrees. During his one and only day on the job, Jefferson never received training on using rest, shade and water to avoid the hazards of extreme heat.

Leadpoint Business Services employs 120 workers at BFI in Milpitas, a tiny fraction of the hundreds of thousands of workers employed nationwide at McDonald’s franchises. But both groups are standing up for a larger cause. Millions of franchise, temporary and contract workers should not be treated as second class citizens. Holding all employers accountable, instead of letting them point fingers at one another, is a critical step in aligning U.S. labor law with the realities of the 21st century workplace.

Mary Vogel, an attorney, is executive director of the National Council for Occupational Safety and Health.