There is a way for workers to avoid being victimised by their employers, in our free market system, by joining a Union. If all workers were unionised then they would effectively control their own work. However, the union movement in this country has become sluggish as the old-timers are all retiring and the young are not as knowledgeable about the benefits of unions. And in addition, what I have seen (im a member of IBEW Local 606) the Unions themselves have been xenophobic, making it too hard for outsiders to get in, so many immigrants are working non-union. Theres alot of complaining about the illegal immigrants taking all the work, and now there is a drive in IBEW and other unions to reclaim that work, but its gonna take more than just voting for whatever democrat is on the ticket (as unions do), because its obvious they wont do anything about illegal immigration except vote to legalize all those here already.
So, anyway, to sum up, the union theory of labor is that when the workers unionise they have more bargaining power as a group if they control all the labor, but this control diminishes as unions allow more non-union workers to operate in their territory. This happens naturally as people become complacent I think.