I understand your point
However, I do think we can draw an important distinction between a slaver or tyrant, and that of an employer.
With the employer, you often have a choice of who you work for. Though I know it's not that simple (I live in a city that is about tied for worst unemployment in the entire USA, so like I get the feeling of being "trapped" at my current job, as there aren't that many available). But ultimately, the worker-employer relationship is usually a symbiotic and mutual one. I've walked out of many jobs in my youth, just being an angry kid. My only repercussion was that the employer no longer had a mutual agreement with me to pay me money. If I was a slave or under true tyranny, and I tried walking out, the repercussions would exceed that of simply ending the relationship. I would be punished.
It wouldn't be the only repercussion, though. I won't quibble too much over whether wage slavery is the same as actual slavery. Obviously, it isn't, and there are differences. Even slavery might be somewhat conditional. A trusted slave in the Emperor's palace might have a more comfortable and secure life than a free man working as a laborer.
As long as other options exist, then people will avail themselves of them, so in your scenario, if the employee doesn't like his job, he can just quit and find another one. We all have freedom of choice, so it should all be good. By the same freedom, workers have had the right to organize and form unions, so that they could have greater bargaining power when dealing with employers over wages and working conditions. That, too, is part of the same voluntary capitalist system.
But when it's allowed to cross international borders, then it could become a problem. If capitalists find that they can't find enough workers for their factory at a certain wage they're willing to pay, then they'll close up and send their operations overseas, where they can pay the workers a pittance under a friendly and easily-corruptible government. Organizing the workers and unionizing them is out of the question, since many of these countries are fascist-ruled police states, so the workers don't have the same rights they would have in the West.
Businesses which aren't really conducive for going abroad take the alternate route of hiring underground, undocumented labor, which is against the law, but ostensibly worth the risk (and with light penalties if caught). Whenever there's political talk dealing with immigration, those capitalists scream the loudest, since they see the potential of their supply of cheap labor drying up.
It never even occurs to them to pay people better wages. They just want the government to turn the blind eye to illegal immigration and other under-the-table practices. They also want the absolute freedom to outsource to any country they want, and they're vehemently opposed to tariffs on imports (that's the real biggie, look at how many people in power oppose tariffs).
If we don't want tariffs, then we shouldn't want international borders at all. Place all countries under a single government, with all citizens having equal rights - and the right to unionize - and then we'll see how much capitalists truly favor this "voluntary system" you speak of.