I'm not sure there is such a thing as free labor. I know I don't work for free. Slaves didn't get wages, but the owners surley had to provide for them if they were to live very long. Not free labor from the owners point of view either. The immigrant labor force in the North wasn't free labor either. Low wages, abysmal working conditions were more akin to indentured servitude.
The fight was ultimately over control of the branches of our government, which tied to the voting population in each state (Compromise of 1850/Kansas Nebraska Act). Political control was the real issue at hand, slavery was just the vehicle (much like Pro-Life/Choice is the political litmus test today). Of course slavery was A central cause of the WBTS, but it isn't an issue that I'm convinced the little man was willing to die for. The WTBS was a "rich man's war, poor man's fight", as most wars are. The WBTS was not, and never has been a black and white, neatly packaged part of our history. It is gray through and through. I believe that is why the likes of us are attracted to it. God Bless America!
Classic logical fallacy. You have invoked the word free under two different definitions of the word, and conflated them to mean the same. Free in the first sentance denotes the ability to determine one's own employment. Free in the succeeding sentances denotes monitary cost.