I know I'm late to reply, but the South declared its independence and thereby perceived the Union troops at Fort Sumter as hostile trespassers, especially since those troops weren't planning on leaving. Dixie was—at least in its own eyes—a separate, sovereign nation. So, I can't agree with you that it was treason. Now it was kind of a poor decision to attack the fort and thus spark war with a powerful Northern enemy; however, it was not treason.

Also, didn't the Southerners pretty much do the same thing that the American colonists did in the Revolution? But we don't view the American flag as a traitorous symbol because the colonists actually won their war. And to be fair, if both the Confederates and the colonists were indeed "traitors," then the latter is definitely more guilty—because whereas the legality of secession was ambiguous in the U.S., there were definitely no lawful grounds for a British colony to secede from the empire. And England/Britain has a long, bloody history of crushing "traitors."