An employer trying to figure out how little they can offer an individual seems like a lot of work, which will blow-up in their face if/when the employees compare compensation packages.
I can't imagine an employer doing this on any sort of large group of employees. Unless you have a mono-sexual, mono-racial workforce, different individual compensation for the same job is just a shit-storm waiting to happen. What if Women are, generally, paid less then men in the same position? Or if minorities are paid less than Caucasian workers?
I've worked in places where one worker ;an older woman) learned she was paid about $5K less than her colleagues, but that was because she came to the job with no experience, the others had 5-10 years industry experience when they were hired. She felt she was 'cheated', it affected her work and her relationships with coworkers. Ultimately they quietly bumped up her pay, but she still complained - it turned out bad for her (impacted her review, cost her a performance raise at year-end).
Bottom line, the worker is owed what the employer offers and the employee accepts. If the offer is too low, don't accept it. It isn't anyone's fault but your own if you accept a too-low offer.
I don't understand the outrage of using publicly-available information to make a business decision - in realpage scandal a company used computers to determine the maximal rent a landlord/owner could charge a tenant, and in this case an employer is using a service to create a profile of a worker from public information to figure out how low an offer the candidate is likely to accept. These are things that have been manually done for decades, but somehow automating it makes it bad?
Employers look at candidates, review their job history, and arrive at a number they think the candidate will accept. That a candidate has gone and used payday loans is (apparently) publicly-available info - the issue is to maybe make the info private?
Employers do background checks, criminal record checks, and, I would assume, some sort of financial background check before hiring certain workers - it's labor-intensive, so probably not very common, but for certain occupations, I'm sure it's standard.