Of course Lee wasn't an abolitionist. Abolitionists were a rarity nationally in 1860 and basically unheard of outside of New England.
Lee's myth is based on the idea that he's a brilliant military commander who took an outnumbered and undersupplied army and kept it in the field for four years, pulling off several battlefield victories against superior enemy forces, and managed to protect Richmond, a capital city a few dozen miles south of the Potomac, for that whole four year period. Lee's reputation is based on his tenacity and tactical cunning, not on his humanitarianism or morality.
EDIT:
I'm reminded of Albert Speer being called a "good nazi".
I think Lee's historical treatment is more akin to Rommel than Speer.