> why is it a bad thing to acknowledge that and try and fix it?

I think the main objection against this kind of affirmative action is that you can't make up for discrimination against one group by discriminating against another group: None of the young, white (and Asian) students that are given a disadvantage today were responsible for the discrimination against black people decades ago – they weren't even around back then.

That's the ethical argument. There's also possibly [1] the economic argument, that this kind of affirmative action is inefficient, i.e., it doesn't fix systemic discrimination, or that all in all it creates more harm than good.

[1] I don't know if that argument is true, but I could see someone making this point.