Libertarians, in general, accept the fact that the state privileges the capitalists at the expense of the workers. Even in the current state-capitalist system, we should not, however, advocate ``labor unions" to ``take the stolen wealth back." We should see labor unions as unethical.
An ethical method to compensate the ``workers" stolen wealth, involves building alternative institutions, not to work within the system. Working within the current system, would not make the workers to have any motivation to ``solve" the problem if ``labor unions" provided the stolen money back.
Working within the system, has another unethical context. As you may know, labor unions, though may raise wages for the majority of workers, causes unemployment amongst the less productive workers. Unions, therefore, constitutes as a form of aggression against the less productive. Using ``labor unions" to take the wealth ``back" equates forcing unemployment to other individuals.
Additionally, besides the fact that labor unions hurts some less productive workers, it also hurts some other innocent individuals. Suppose if some individuals work for a capitalist, who, coincidentally, does not privilege from the state. Using labor unions against all capitalists, therefore, may hurt the innocent capitalist who does not privilege from a state. We should consider labor unions as a form of collective punishment, and we should never use them as a form of ``defense" against the massive robbery. This matches with the libertarian ethical claim: ``As long as we hurt innocent individuals in the process, this constitutes aggression, and never constitutes as defense." We, thus, may consider labor unions as ``gangs" or ``cartels."
Some dispute ``aggressiveness" as an inherent attribute of labor unions. They defend the that voluntary types of labor unions may exist in a free society. Yes, we should not have any problem of them within a free society, but unions, in a free society, would probably not exist too much. Co-operatives and self-employment would replace labor unions, for example. Additionally, labor unions, even in a free society under the situation of corporations employing workers, might not even increase the wages of the workers, because of greater competition between employers. So why should we advocate labor unions in the current statist society, while, in contrast, in a free society, labor unions would probably not exist to a significant extent? We should never advocate them, as it would probably imply using the non-voluntary types of unions and as ``working within the system."