I do not think this post was questioning a general display of The Confederate Flag. There are two instances that it could be illegal. 1) To incite hate. 2) Anywhere that it is beholden to The Civil Rights Act. [Such as the workplace.] The Prosecution would have to prove that the flag is being used to intimidate or discriminate.
Additionally,a lot of public places (malls, stadiums, etc.) have rules about what you can display or wear. You can ask to see the written rules. Or they may give it to you when they call security to remove you.