Plea bargaining is common in the US, with defendants often able to secure a more lenient sentence if they admit an offence and co-operate with prosecutors, rather than contest the charges in a trial.
We who live in the world of bargaining and negotiation, compromise and common sense, know that ideals have to be funneled through the dirty dharma of daily life.
The case for this law is pretty common sense: knowing what their coworkers make will give employees bargaining power, and may even prevent discrimination.