Why do liberals instantly see the confederate flag as a sign of slavery?

when the war was about states rights and only lincoln, in a mere war act, inadvertently caused the flag to be viewed in a negatively racist manner, fueling more racism years down the road? the north held slaves even as lincoln signed his name and even himself stated that blacks should never be allowed to hold public office nor be allowed to vote....being that the dead are dead and the page of time has washed away all of the untruths, to a degree...why does the north still cling to this ideal like a piece of flotsam and jetsam from a battle won? the north was not anti-slavery and there was much protest from it against the war.