You are wrong on your facts and understanding of the law. The Confederates absolutely did commit treason. The Constitution does not allow unilateral succession; and states do not, and have never had, a right to succeed from the Union. This is settled law (see the 1868 Supreme Court case of Texas v. White).
The Confederates were racist traitors fighting for a despicable and illicit cause. Any statute glorifying a Confederate is an affront to this Country and its true soldiers. It is an affront, not only because they were traitors to this Country, but also because their cause was abhorrent and contrary to what America purports to, and in fact does, stand for.
These Confederate statues were mostly erected in the 20th Century; and despite all the “preserving our culture” dog-whistle nonsense, the people in power erecting the statues largely did so for the purpose of sending a message to blacks that “we” (the white supremacists) are still here and in charge. But, whatever the professed reasons for these statues, it doesn’t matter. Statues paying homage to the Confederacy have no more reason to be part of a public display than a statue put up by a state or local government to celebrate Nazis who fought against this Country in WWII. The causes of both groups were despicable, and both were traitors.