Salmon Chase, the CJ of the USSC, and an abolitionist and former member of Lincoln's cabinet (so no friend of the South) recommended against prosecutions for treason and warned that there was no legal authority under the Constitution and laws at the time. The reason for this is that citizens were "citizens of the several states" and not of an entity called "the United States." That came much later.
Even Jeff Davis was never prosecuted. They kept him imprisoned for awhile but eventually let him go, because they couldn't make a charge stick.
Several New England states had attempted to secede, unsuccessfully, some time before, but with no penalty or charges.
His ruling was to re- unify the Country.