Yes, there are reasons for differing salaries and they have important functions. But at some point those justifications stop to make sense and it is really just about exploiting your position to grab as much as you can for yourself.
How is demanding your fair market value exploitation? Do we really need to take pity on companies grossing massive incomes while simultaneously not paying their fair share of taxes?
Has there ever been a union that pushed for lower compensation for it's members? A union advocating/negotiating for as much as it can for its members is exploiting its position in the same way.
No, trying to get a better compensation is not per se the issue. In a hypothetical company where the management takes home all the profits and the workers make minimum wage, moving to a more fair distribution of the profits involves less money for the management and more money for the workers.