The Confederates are essentially called "rebels" by the fact that they broke away from the union to make their own Sovereign state, namely the Confederate States of America. So during the talks of surrender, Grant made a generous offer. He offered that Lee and his men would NOT be tried for treason under the condition that they turn in their arms. He also allowed them to keep their personal belongings which in a situation such as this, a betrayal of a Union, is almost unheard of. This allowed Lee's men to return home and work on the rebuilding of the South without any fear of repraisal.