Earlier quoted context omitted.
How so?
A manager could then point to the salary of someone in the same position and say "Jane gets paid $X in the same position, it would be unfair to give you a raise and not her" I don't work somewhere with transparent salaries and I already get that excuse — I've more than once heard "you're kind of already at the top of our range for X position" and "if we pay you anymore you'll be making more than me!"
It sounds like managers will be doing that either way. With open salaries they at least wouldn't be able to lie about it.