Defending the Confederacy and/or its symbols, by pointing out the flaws of the United States is an informal fallacy: tu quoque, attempting to refute a position by pointing out its hypocrisy. In short, one is arguing that the Confederacy’s raison d’être is okay because the United States also has blood on its hands. Let us be clear, the States forming the Confederacy seceded because they wished to preserve the institution and practice of slavery. You, dear reader, needn’t take my word for it feel free to read the declarations of secession of Georgia, Mississippi, South Carolina, and Texas; not to mention, that the Confederate Constitution is quite clear about the importance of slavery in its mission.
By and large, the same persons that defend the Confederacy as not being about slavery are the same ones who deny the existence and lasting effects of the darker side of US history: slavery, Jim Crow, genocide of the indigenous population, subordination of labor to capital, etc. (this list is non-exhaustive and merely points out the crimes that were largely concurrent with the American Civil War era). The prime difference between flying the US flag despite its crimes and flying the Confederate flag is that the Confederacy existed because and stood in defense of “the peculiar institution”.
The US on the other hand despite the best intentions of its propertied masters, alias the Founding Fathers, accidentally conceived a nation founded on an idealistic principle of equality for all which causes its People to act in such a way as to attempt to actually create such a society.