It is no more a banner of treason than the United States flag. Every major empire throughout history started as treason. If it is successful it is branded a revolution if it fails then it is treason. While it is easy for you now with a different upbringing, way of life, culture around you to say some of the things the south fought for were wrong. Had you been a white male in 1850 you almost certainly would not see it as wrong.

It has been used by groups as a symbol of hate (which is their right). They have just as much right to their opinion as you or I. That does not mean that the flag itself is inherently hateful. It also does not mean it represents hate to everyone who waves it. I would assume you would not want the Koran banned because it has been used to justify evil actions? The 20 dollar bill is still in use with little to no thought of the atrocities Andrew Jackson committed against Native Americans.

The most important reason is banning a flag is a very slippery slope when it comes to our right of free speech (if not a nose dive off said slope). No matter how you feel about something every person in this country has the right to feel the opposite and express that. It does not matter if you like it or not.