The logic is straight-forward. Whether it is necessary, correct, or advantageous, depends largely on your circumstances and personal put look.
A) historically a number of groups have been disadvantaged.
B) this has lead to those groups "falling behind" advantaged groups from a wealth perspective.
[side bar - wealth begets wealth, while poverty begets poverty, and while it is possible to rise up it is hard]
C) since it is easier to get good grades when you have money, it's more impressive to get good, but not necessarily top, grades from a different starting point. Thus the route, not just the result, should be considered when reviewing applications.
D) thus admitence with "just grade scores" tends to favor those groups with money, which would result in a largely hermogenous student body.
E) education extends beyond just knowledge. The goal is to turn out adults who will function well in a society as a whole, get good jobs, add value to the community, and do on.
F) in modern times it is considered poor form, or worse, to exhibit bad speech or behavior patterns. You can no longer behave like your great-grandfather, you can no longer speak like your grandfather. To do so will harm your prospects.
G) thus, in order to become better human beings, its good to rub shoulders with those of different social, financial, gender, ability levels. In the safety of a school environment everyone can better be taught, and practice, the social norms of the day.
H) thus a diverse student body is good for all students. Previously disadvantaged groups get a leg up. Currently advantaged groups get to interact with those dissimilar to themselves, and thus improve those interaction skills.
US society likes to believe that "hard work" is the secret to financial position. But hard work itself is hard to measure, so we measure grades instead. However grades are more a function of current economic status, not work, and if anything reflect the hard work of the parents, not the students. There are various tools used to try and improve the proxy measure, things like diversity quotas and so on.
Of course imperfect improvements to imperfect proxies still result in contraversy since the system is still flawed. And naturally those most disadvantaged by the rules feel the sting the most.
None of this explains or excuses the behaviour of the school in with-holding these results though. The logic behind that is strange. Why would a top quality school, with a diverse student body, look to prejudice the ability of those students to get good college positions?
I can see some logic in not making a public song and dance about it, but surely all students, and their parents, should be notified as soon as possible?
On the up side, the fact this is "news" at all suggests this is a very uncommon practice.