You assume that more information asymmetry always helps the employer; it is also possible that a lack of information makes the employee think they're getting a 'bad deal', because they overestimate how much their coworkers make (possibly due to something akin to impostor syndrome). If Amazon thinks they can find someone of equal skill for less money, they should make the change, and if the employee thinks they are worth more than they can get with Amazon, then they should leave; I really don't understand your (second?) point.

I never said the above-median wages had anything to do with national figures; I meant that depending on what the employee and employer were bargaining around (more likely to be company median wages than national), the comparison would help the employees bargain up half the time, and the employers bargain down half the time.