There's no such thing as "free" labor. I believe "cheap" labor is what's clearly implied here. The modern-day slave equivalents are the immigrants that come into the United States in search of a better life. After having immigrated into this new land of "endless opportunity" they work for poverty wages and often times do not share full citizen rights. Furthermore, they face all types of social discrimination for being an outsider. New immigrants become the lowest cast of any society that they immigrate into.
Through this process of immigration, they elevate the previous lowest cast group into a higher economic stratum, since the new immigrants take up all the dangerous, back breaking, soul crushing low wage slave work that the previous generation of slaves were responsible for.
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An example:
Let's say you are an unskilled, poor white American working for poverty wages . You book a 2000$ roofing job, pick up 4 Mexicans hanging out near your local Home Depot and agree to pay them a 100$ for a days of labor. You pay your slave labor 400$, and their unpaid wages($1600); meaning their time, blood, sweat becomes your profit. Just as a Vampire sucks the life energy of its victims and makes it, its own. So you in effect suck the blood, sweat and pain of your slaves and make make it into your prosperity.
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Ugly, low status males built the entire world.