[zo1]

Maybe I'm missing something... If the data doesn't come from the employer, then how does the "payroll" company get it?

[jacobr1]

It is indirect instead. The employer tells the payroll company: pay employee $x (and handle deductions, retirement contributes etc ...) The payroll company then has this data and can resell it. I would expect the contract the employer has with the payroll company explicitly allows for the data sharing.