I have heard this claim on many talk shows, including Thom. No, he was not a traitor. Be careful how you label same.
Washington and the signers of the Declaration of Independence were declared Traitors of the Crown by King George as well, but is forgotten in History
Lee was an honored West Point Graduate. He was respected as a leader on both sides of the battle line. He lost. He not only lost, but surrendered to the opposing force with honor and dignity. Traitors are usually hung from the highest tree, or convicted of war crimes, then executed. That did not happen after the Civil War.
Was Sherman convicted of abuse of war powers with his march through the South? How many innocents/infrastructure perished because of his march?
Were mistreatment of POWs (on both sides) during the Civil War brought to justice? A: no.
Was Barron Von Rictoven (WWI pilot) a traitor? When he was shot down, he was given a military funeral, honored by military from both sides of the conflict.
During the Gulf War, captors asked why a tank commander had a picture of Rommel inside his Abrams Tank. "You don't know history, do you?" was the reply.
Today, there are screams from the left for California to leave the Union. Would they be traitors? On the Right, there are many in Texas that want to do same. Would citizens (or lawmakers) in both States also be declared traitors of the US?
I'm a transplanted Yankee now living in the South since 1959. I have seen (and abhor) the Back of the Bus signs and dual water cooler/restroom signs. Also glad to see the removal of the Confederate Flag from SC Statehouse grounds, but to remove monuments to eliminate history should be thought out carefully before doing same.