Preferential treatment isn't even what's being discussed.
Preferential treatment is exactly what is being discussed. A biased test is not equal treatment any more than segregated bathrooms are equal treatment.
IQ correlates powerfully with one's ability to be successful in general.
Successful inside the confines of a specific culture, with specific biases reflected by the IQ test.
It correlates with health, sperm quality, job productivity, the IQ of the spouse, body symmetry, IQ of the kids, education, brain size, problem solving ability, reaction speed, inclination to being law-abiding--just about everything.
Just about everything you list there - spouse IQ, job productivity, education, problem solving ability, inclination to being law-abiding - directly exhibits the same cultural biases as the IQ test, so that's not telling us much.
As far as the other factors go (health, sperm quality, body symmetry, brain size, reaction speed): sure. Nobody disputes part of what IQ tests measure are genuine attributes. That doesn't imply the test isn't biased. Moreover, we'd expect all of those physical attributes to be depressed in minorities to begin with, owing to generations of oppression. Which is to say that, to the extent that minorities do perform more poorly on unbiased tests, that is direct evidence of the continuing effects of their historical mistreatment. To get to a place where you can point to poor minority achievement on tests as anything other than racism in action, you have to establish both that the test was unbiased, and that the people taking the test weren't inherently disadvantaged by past racism.
Which is what gives rise to all of the objections here: people are insisting that tests are unbiased, and past/current oppression negligible, without any supporting evidence or argument, and in the face of substantial contrary evidence and argument. If that's not racist bigotry, I'm starting to wonder what is.
That's life. Everybody is forced to conform.
But not everyone gets an equal say in what standards everybody is forced to conform to. Hence the disparate outcomes, and unfairness.
Minorities should not be exceptional
Indeed, they should be granted the same agency as white people to influence the cultural bias of tests, and society at large, so that they are not unduly disadvantaged by them. Which is exactly what is at issue here.
... and... how, actually, is it that we've determined that BIAS that's causing them to stink at the test in the first place?
The lack of any more plausible explanation. Minorities excel in police jobs in the same department that are not subject to this test, so it's difficult to believe they're inherently unqualified to hold the higher positions. In positions subject to the test, a vast racial disparity is observed, that does not occur in departments that use different criteria for promotion. Hence, there are substantial grounds for presuming that the test is biased. At the minimum, the burden is on any advocates of the test in question to justify it. And we have seen none of that, so far.