Mary Jo --
In actuality this man surrendered twice -- once on Lookout Mountain, and again at Rock Island. If you see a copy of the oath of allegiance he was required to sign (much like the pledge of allegiance children used to repeat in school), you'll understand. They didn't just open the doors to the prison!
Here's a link to Dunbar Rowland's history of the 24th Mississippi --
Rifle pits defended by these men on the side of the mountain are still visible near Craven House today.