They can afford to pay those who negotiate closer to their true value (often still significantly less) is because they pay the vast majority of people less.
The only way transparent salaries help the majority instead of being used as a tool for businesses to just say, "We only pay 100k/yr, that's what everyone makes" is if there is some body enforcing fair pay.
The only way transparent salaries help the majority instead of being used as a tool for businesses to just say, "We only pay 100k/yr, that's what everyone makes" is if there is some body enforcing fair pay.