Uh...yes it is unreasonable.
It was discussed and rejected at the time.
See: https://www.civilwarprofiles.com/grant-protects-lee-from-treason-trial/
If you read the Surrender Document and the Parole Document neither specifically protects the rebel soldiers from treason charges after the rebellion. But Grant did believe that such protections were implied and he was willing to go to the mat to protect Lee and others. It was a most honorable position to take, and Johnson eventually came around to that way of thinking when he issued his amnesty proclamations.