I'm not too familiar with the US justice system, but I read the Wikipedia page the last time you brought up that specific case. Reading (a little) between the lines, Duke Power seems to have set out to discriminate against black employees. That seems very different from using an IQ test to select the best employees, especially for a job that actually is highly IQ-dependent.
It takes one bad apple to ruin it for the bunch, as they say.
That said, I lived and worked for a few years in a country where IQ tests were the norm for hiring people. When hiring for my own team (C++ devs), I did not use them as I found they were a very poor filter with both high false positives and high false negatives in terms of indicating whether or not you should hire a particular person.
Intent is often irrelevant. If something is shown to have disparate impact on a protected class then it is likely to be ruled discriminatory, and therefore illegal, regardless of intent.
