The Work Number's entire sales pitch is... "We'll accept the data from you for free. You can give our our number and website to anyone who calls into your HR looking for employment reference, employment verification, etc. We'll field those calls for you."

They don't pay for the paystub data. The employers give it to them.

Although it's an invasion of privacy, to be sure, it actually does have some benefits for the employee.

In places outside San Fran, where people actually get conforming mortgages, having your data in The Work Number's database automates and cuts out the employment & income verification so that you don't have to track down records and submit manually and can potentially skip multiple must-connect phone calls between the lender and employer.