Generally speaking, slavery is also benefitial to both parties, you're either a slave out you get killed. While technically voluntary (because a slave can still choose stand up to the oppressor, even if it's guaranteed to fail) we don't consider slavery voluntary. We can say that in this day and age our work is voluntary, but it's debatable.

You can look to this year how "voluntary" it is when the Hollywood execs literally said they will wait for the protesters to starve so they'd get back to work. When there's such a severe power dynamic it becomes almost no different to slavery, because you, individually, can be effectively forced back to work. The only reason Hollywood protests have any chance to have impact is because they collectively oppose the oppression. The power dynamic is being balanced (or dipped in the favor of labor) by sheer number of protestors / workers.