At that time in our country, the confederacy was looked upon kindly by the left because it represented "resistance" to the Federal Government. The Federal Government was sending boys to Vietnam, therefore the Federal Government was unpopular.
I went to a college in NYC. I saw a lot of demonstrations. I saw vietcong flags, north viet flags, upside down American Flags. I never saw a Confederate Battle Flag. The Confederacy was personified by George Wallace. Not really a hero of the Left.