Is it your contention that the people defending the statue of Lee would have given similar reasons for their behaviour as those who restored Auschwitz's sign? I ask, because that seems very unlikely to me.
Context is important. Few are arguing that there shouldn't be remembrances of those people and events and the reasons for them, it's how those things are remembered that is in question.
No, but the reason is less important than the pros of this outcome.
Would you condemn a man who saved a child from a burning building because he wanted his 15 minutes of fame and not because it was the right thing to do?
I'm deliberately using a hyperbole to emphasize a point, but there's also a much more recent example of statue destruction: ISIS has destroyed a lot of statues. Is this the company one should keep?
Note that when statue destruction is normalized, broken windows theory kicks in and statues of prominent black civil rights activists[0], Lincoln, Mahatma Gandhi[1], etc. get vandalized as well. I think very few people would agree that Gandhi (I'm not a scholar of history, but I've seen many people referencing him as arguably one of the most virtuous people ever) deserved this.
Context is important. Few are arguing that there shouldn't be remembrances of those people and events and the reasons for them, it's how those things are remembered that is in question.