My assumption has been that they'd attempt to infer by marrying the two pieces of info whether it bit you'd pose a risk. So say you have a fraud charge laid against you some years back and you're currently in heaps of bad debt... It might be a bad idea to let you at my financials database, for instance.
I do think though that some of the more stringent assessment procedures also serve a secondary purpose when it comes to offer time. If you've run the gauntlet to get this offer the person offering you the position has considerable leverage in terms of negotiating a remuneration package. May it's not the main reason they do that stuff but it is a side effect of it.
Once I had the permit I spent 10 months in KwaZulu-Natal - an area with 40% HIV incidence, and at an employer with (anonymously tested) 20-25% incidence.
The acronym CCMA is thrown around a lot depending on whose conversations you listen to.
Credit history is a mostly used as a poor indication that you won't be tempted to sell company or user private information. It might also be an indicator of poor judgement or other bad traits, I guess. I think only one company ever asked this of me, and they were certainly the most backward of those I interviewed at.
Basically, companies are trying to avoid hiring bad people. Whether it's because they've bought into the "it is impossible to fire anyone" story or not, it is costly in terms of time and wasted effort and disruption. And, generalising from an admittedly small sample size, I'd say they're not as good or as introspective about how they hire as the few (admittedly really good) US companies I'm familiar with.