This is definitely true, but we can go a step deeper. You have to think about what Harvard's goals are. Maximizing academic success is not very high on the list. The real goals are maximizing career success, donations, and cultural cachet/prestige. When legacies are considered with these goals in mind they make a lot of sense. Legacies are likely to have successful careers due to their parents resources and power, they're likely to donate too because of all the resources they have.

So yes, legacies are the main value add at Harvard type institutions for the non legacy students, but even if they weren't, admitting them aligns with the universities goals. If we're considering the "fairness" of admissions we have to look through the perspective of what admissions is trying to accomplish. We can get into equity vs equality, but at the end of the day accepting legacies does probably maximize Harvards chances of achieving its goals, and many would say that makes the process fair.