I see not-governments cartel everywhere, especially when the governments doesn't regulate them.
And slavery is not a fair exchange. Guess what? Governments stopped slavery.
Why a capitalist should free a slave if he can exploit it?
Heh, government hasn't stopped slavery.
A slave can produce more with an illusion of freedom. Chattel slaves are horribly inefficient. I get a whole lot more by taking a cut off his labor through a tax. In fact, I can enable this form of slavery in large populations through positive-reinforcement through entitlements. Happy, happy, happy slaves and more money for me.
Workers that fight for their rights and form unions and what else aren't very efficient.
That's why a lot of industries moved to china or other poor countries (now they are moving AWAY from china, since it's no more so poor), to exploit their cheaper workforce with less rights.