[googlryas]

Candidates do essentially have to show their cards - employers verify your salary history with products like "the work number".

Most people don't even know their salary is being reported by their company to a third party, which then packages and sells that data.

[WalterBright]

I thought there was a law in some states preventing that.

[googlryas]

Yes, but it is less than half of the states(though I'm not sure what percent of the population are covered), and even then, many of the bans are phrased such that pay history questions are only banned before an offer is made, or it is voluntary but employees may not know their rights.

It is still legal to rescind or modify an offer after you discover salary history in most states with a "pay history" ban in place.

https://fitsmallbusiness.com/salary-history-ban