Actually it was not treason. States are sovereign and have the right to unilateral secession. That’s why nobody - not even Jefferson Davis was prosecuted for Treason.
“If you bring these [Confederate] leaders to trial it will condemn the North, for by the Constitution secession is not rebellion. Lincoln wanted Davis to escape, and he was right. His capture was a mistake. His trial will be a greater one.” Chief Justice Salmon P. Chase, July 1867 (Foote, The Civil War, Vol. 3, p. 765)
“If you bring these leaders to trial, it will condemn the North, for by the Constitution, secession is not a rebellion. His [Jefferson Davis] capture was a mistake. His trial will be a greater one. We cannot convict him of treason” Chief Justice Salmon P Chase [as quoted by Herman S. Frey, in Jefferson Davis, Frey Enterprises, 1977, pp. 69-72]
I’ve seen that quote cited several times, yet have never been able to find where it originally came from. I find it highly unlikely that the man who said this; “Constitution, in all its provisions, looks to an indestructible Union composed of indestructible States.” as the Chief Justice of the supreme court would have stated secession was not rebellion.
I have found that Chase did not want Davis brought to trial, but not for the reason the quote states. He believed that the 14th Amendment already punished Davis, and the other rebel leaders, by stripping them of their voting rights. So it would be double jeopardy to also convict him in court.
Case of Davis, 7 F. Cas. 63, 102 (C.C.D. Va. 1867)
Nevertheless, the fact still stands that the southern rebels were pardoned for committing treason by Andrew Jackson. And that Davis was pardoned by President Carter in 1978. So they might never have been convicted of treason, but they did commit it and were pardoned for it.