You answered your own question really. I also find people respond less aggressively to that smiley
But to the point, shouldnt the value of labor be decided by the two parties directly involved, not a third far removed from the situation? If citizen A voluntarily agrees to perform a specified task for citizen B at a specified rate. Where exactly has the evil/injustice occurred?
That's a vast oversimplification of how wages work in the real world...but then again you could say that about Libertarianism in general.
Main problems:
1. Very often one party (usually not labor) is much more powerful than the other, and there's often little incentive for companies to do anything other than pay their workers as little as they can without said workers leaving. This is especially true in an economy with high unemployment.
2. "Voluntarily" is relative. Is it really voluntary if your options are "Work for whatever I feel like paying you, or go hungry"?
But let's go back to the two parties thing -
If we really want the value of labor to be a fair negotiation between workers and a company, then the only way workers will have a real influence would be to negotiate as a group. So with that said, where do you stand on unions?