Imagine, if you will, a
relay footrace between two teams.
Upon starting the race, it becomes apparent that the runner one of the teams is carrying a 50-pound weight on her back, and being forced to run through knee-deep water. And that runner is lagging behind badly.
And after the first change of runners in the race, the one team once again has their runner handicapped. The weight seems a bit smaller, and the water a bit more shallow, but they're still suffering.
Finally, a bit later, they seem to have decided to make things fair. The one team now no longer has to deal with the weight or the water. But they're still at quite a disadvantage, because the other team got such a head start.
So how do we make the race, which is still going on, fair again? Especially since each individual runner on the team is judged against the corresponding one on the other team? If you stop the runner on the advantaged team, is that fair? Especially since the runner didn't do anything wrong, it's not her fault she started with the advantage, and she doesn't think waiting for the other to catch up is fair.
If you try and do anything to even it back up, the runner on the one team suffers for something she had no part of. If you don't, the other team has to be much better to even catch up.
This is the difficulty when dealing with affirmative action. African-americans got a really crappy start because they were handicapped in every way possible. But if you try and make it up now, people who had nothing to do with putting them down suffer. It's a no-win situation. Treating everyone equally now doesn't eliminate the effects of past injustice.