Man A is abusing Man B's ignorance of his own worth. Man A is also colluding with his competitors to make sure Man B can't make more anywhere else. Man A is also always, always, ALWAYS, supressing Man B's opportunities to unite with his fellow workers, either by union busting, or by lobbying to destroy the education system that would allow Man B to inform himself of his actual worth.
But I guess if we only look at billionaires with a 2 dimensional perspective, and ignore all the things they do to make sure their employees couldn't possibly demand more money, then billionaires look like they're just giving people job opportunities...
But if they paid their employees their worth, they may only have money for the next 20 generations instead of 40....