How bizarre, this seems economically unjustifiable, that is to say— I smell a rat.

The rich are just suspicious of the poor. They trust people from similar backgrounds as themselves. Employers irrationally think they can gauge a person's trustworthiness just by interacting with them for a few hours during an interview. And trustworthiness is virtually impossible to test for in higher paying roles: people earning low wages ususally don't have the power to defraud the company too much, but there's a lot of people with that limited power so they're a threat. Wealthier employees could engage in petty fraud too, but they have less incentive to do so because they don't need the money as much, and there's not as many of them, so even if they did it would be less damaging. Instead the concern over richer, more skilled and educated employees is that they'll steal large amounts from the company, and unless they end up in prison, nobody will ever know (contrast with working-class fraud which will usually only end up losing the person their job, so there's less incentive to avoid being caught).

There's probably no conspiracy, just generalized classism.

To generalize: it's similarity/ingroup bias - as a human you are biased to be more sympathetic to people like yourself.

It takes a lot of empathy (something that is actively selected against in our corporation dominated workforce) to bypass that.

Add into that the gravity of money (ie, rich get richer) and political corruption (money buys power) and you have a basic framework for classism.

Are there some people who exhibit the trait of being biased toward people who are not like them?

Believe it all comes down to insurance and having someone else to blame. Think about why most jobs use urine instead of hair for drug testing. Hell of a lot cheaper, don't care about accuracy, and just want to be able to say that they did it.

The costs for hiring are enormous, and the cost for lost revenue are big for hiring agencies.

If I want to farm your labor, I need to find you, make sure you are going to work long enough at a site for me to make money, and I have to hope that you wont do something that will sour the relationship I have with "my" customer (the business I am renting you to.)

The margins for staffing places are actually pretty small per person, so its not so much unjustifiable as it is dehumanizing.

So why increase your costs using data that has no appreciable correlation with what affects your bottom line?

I worked temp out of college in the dot com bust. I lived out of a borrowed car. I was desperate for work and had relevant skills, but many temp agencies wouldn't touch me. It was stupid and dehumanizing, but it was also pretty unjustifiable.

Because it shifts perceived responsibility for any failures from the temp agency to the service that performed the background checks.

Except that the BC folks would never accept such responsibility, and the temp agencies require employers to waive the right to claim (limiting the damages to reputation).

So maybe that is the logic, but it's specious logic.

Nothing compared to the reputation damage and fallout when something happens and the client is incredulous. "You hired a felon?"