The problem is that this history is rooted in racism: the Confederacy was founded to preserve the institution of slavery (people who claim otherwise need to read the states’ own declarations) and the Army of Northern Virginia battle flag most commonly displayed has a long history from the Jim Crow era of being used to intimidate free Black people.

Just as someone proud of being German has a wide variety of symbols they can use other than a swastika, someone proud of their Southern ancestry could pick almost anything else to express that. Choosing a symbol which racists have proudly used to intimidate others for a century in a half immediately calls motives into question and even if someone somehow managed to avoid learning the history until now the only polite response is “I had no idea. I’ll use something else!”