One of the fundamental splits between libertarians and left-liberals is the two groups’ understanding of voluntariness. For libertarians, voluntariness is a purely procedural notion. Any agreement made without active force or fraud counts as voluntary, regardless of what else obtains about the agreement. If you are on the verge of starvation and you sign an agreement promising 50% of your future earnings for a piece of bread, this is considered voluntary.
The left-liberal side of things understands voluntariness in a less procedural way. Even when active force is not present, power structures within society — be they economic, gender-based, or race-based — affect the genuine voluntariness of any given agreement. On this view then, agreements are not completely voluntary or completely involuntary; instead, they are more voluntary or less voluntary depending on the circumstances of the agreement. That is, voluntariness is a spectrum, not a binary.
This is the theory at least. In practice, libertarians often slip into left-liberal understandings of voluntariness when it is convenient for them to do so. Consider income taxes for instance. Libertarians claim that income taxes are theft because they involve government force. But in a purely procedural sense, that is not true. Whenever someone enters into an employment arrangement, they agree to have the employer withhold income taxes. They are not forced to make this kind of agreement. If they do not want to make it, they can refuse to sign the agreement and not work at all.
Sure, they will die if they do not work, but that is not a relevant consideration under libertarian ideas about voluntariness. Recall in the first paragraph where voluntariness was said to exist when a man on the brink of a starvation death agrees to an exorbitant price for a piece of bread. This is counted as voluntary because the man has the choice to simply not buy the bread and die. With income taxes, people have the choice to simply not work and die. No one forces people to make the withholding agreement; so, they must do it voluntarily.
A libertarian might come back and say that the employer is being forced to withhold taxes. If the employer refused to do so, then the government would use force against them, and so that is where the involuntary taxation comes into play. But this too is not true. The employer has the same choice the employee does: they can choose not to run a business. When they get licensed to run their business, they agree to follow the taxation laws. If they do not want to, they certainly have that option in the form of not opening their business.
A more level-headed analysis would point out that not working and dying is not really an option, at least not in any normal sense of the word. But that kind of nuance puts you outside of libertarian ideas about voluntariness and into left-liberal ideas. And once you have entered into the left-liberal realm, everything else about the libertarian position on voluntariness falls apart. One cannot say, for instance, that a worker agrees to the horrible conditions and horrible pay that they receive if their only other option is to withdraw from the labor market altogether and die. Yet that is precisely the situation of the wage worker in a laissez-faire capitalist economy.