Prisoners in more than 20 states went on a coordinated strike Friday, refusing to go to their assigned jobs and demanding an "end to prison slavery." The work stoppage, staged on the 45th anniversary of the Attica prison uprising of 1971, marks one of the largest attempted prison strikes in decades.

"Slavery is alive and well in the prison system, but by the end of this year, it won't be anymore," reads a statement by the Industrial Workers of the World's Incarcerated Workers Organizing Committee (IWOC), the group that put together and announced the strike. "This call goes directly to the slaves themselves."

It may be no surprised that America's 2 million-plus inmates mop floors or scrub toilets in their prisons and jails, but the country's prison population is also a source of cheap and, in states like Texas, even free labor.

As state budgets have shrunk in recent years, prisons have launched new work programs. Prisoners repair public plumbing, clean up roadkill, manage public spaces such as graveyards, and even do underwater welding.

"Think about how much it costs to incarcerate someone," Republican Senator John Ensign said in 2011, advocating for more of these programs. "Do we want them just sitting in prison, lifting weights, becoming violent and thinking about the next crime? Or do we want them having a little purpose in life and learning a skill?"

The true scope of prison labor goes beyond personal development and public works: It's good business. Prisoners scrub products for Wal-Mart, package coffee for Starbucks, sew clothes for Victoria's Secret, and man call centers for AT&T.

Corporations cut deals with both private and public prisons, which gives them access to a labor force that has no choice but to work for, say, 20 cents an hour. Businesses and governments in turn save big.

"At Holman prison, the industry we have there is the state tag plant, which produces license plates for the state, and also a sewing factory," said Robert Horton, public information manager for the Alabama Department of Corrections. Horton added that operations had not been interrupted by the strike.

Prisoners are not permitted to unionize, as labor law does not consider them employees. That's something the IWOC is looking to change by clandestinely recruiting prisoners to join up with a guild free of union dues.

"You cannot change this situation through a grievance process that doesn't work... or through courts that are clearly against you... or through petitions to lawmakers who don't care about you because you don't vote... or through hunger strikes against prison officials who want you to starve... or through letters to newspapers who have ignored this situation for decades," the group's site says.

"We know what will happen if you DON'T join the Industrial Workers of the World. Let's see what happens when you DO."