The Confederacy offered freedom to any slave who chose to fight for the Confederacy in the Civil War, before it was so offered by the Union.
The primary issues were State’s Rights not being overwhelmed by federal power.
One could charge Union soldiers for being traitors of the US Constitution for not respecting the rights of other states.
A war was fought over the issue. Had the Union been just, as victors, they would not now try to rewrite history to claim the war was premised as one against slavery. They simply cower from their past unjust position and seek to pivot towards more recent political hyperbole.
Please cite the documents issued by the Confederate Government that offered slaves freedom in exchange for service in the Confederate Army. How many slaves took the Confederate Government up on that offer of freedom in return for service?
As far as charging Union Soldiers with treason, read the definition of treason in the United States Constitution.
Article III section 3 of the United States Constitution states “Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort.” It would be much easier to levy treason charges against General Lee, General Bragg, or General Gordon then it would be to charge Sergeant Bull, Private Billings, or Corporal Klegg.
Had the South been honest in accepting their defeat, they would not have tried to rewrite history to claim “States Rights” as the premise for secession. They simply cower from their past unjust position and attempt to deny the real driving force behind the secession of the first seven states.