Having been a manager once, pay seems such a gray area, since people negotiate it with their manager. How does this law distinguish between unfair pay practices

Having been a manager once, pay seems such a gray area, since people negotiate it with their manager. How does this law distinguish between unfair pay practices and the myriad non-discriminatory reasons why someone might be paid less? Are employers supposed to be responsible for the poor negotiating skills of a candidate? I want equal pay, but unless there is damning evidence of paying people less because of their gender, I am not sure how I feel about this law. Also this blog seems very angry politically polarizing, the kind of political blog I always hope to avoid reading.