Serious question: I really did think that the celebration of General Lee to this day had to do with his military prowess, not his alleged kindliness. How is Lee different on this front than his direct contemporary Helmuth von Moltke the Elder, the legendarily effective Prussian/German general widely celebrated as one of if not the greatest general of his generation?
von Moltke the Elder was no winner on political principals, but the man knew how to win battles. I always thought the celebration of Lee was about his military competency in winning engagements like the Battle of Chancellorsville which Lee had no business winning against a Union army more than twice the size of his, and not due to Lee's actual character as a human being.
Lee is celebrated as a general, but there also is the notion that he was a cut above as a man as well. To liken to a more recent figure who was respected by his opponents despite serving a hideously depraved regime, the attitudes I have seen towards Lee are similar to what I have seen displayed towards Rommel.
With that said, I have also never heard anyone allege that he was an abolitionist. In fact, even the views suggested in that letter, as bad as they are, would have been considered outright progressive by the standards of the Antebellum South. I have recently been reading the autobiography of Frederick Douglass and many of the Southerners he describes encountering say far more viciously racist things than that.
