Public income info means it is much much easier to negotiate against HR. You know exactly how much x or y makes given a or b. This is info that HR has, because

Public income info means it is much much easier to negotiate against HR. You know exactly how much x or y makes given a or b. This is info that HR has, because they pay for it. If it was public, you'd have it too. Your bosses also would not be allowed to guilt-trip you into foregoing a raise without them taking on a comparable part of the burden. Salary secrecy seems like a quintessentially american thing, completely at odds with faith in markets. If you believe in markets, let the labor markets do the best job they possibly can, by informing agents within them and tearing down information asymmetries.