It was a different time, and a different sensibility. I do not doubt that they cheered the courage of the Union troops even as they gunned them down. Many gallant gestures were made in those days, and many ruthless ones as well. The top officers on both sides had mostly served together as friends in the war against Mexico. The cameraderie they had experienced then gave them more than enough reason to appreciate the humanity of their opponents across the line, and that would have communicated itself to the ordinary soldiers too.

Similarly, in World War II, the Japanese (who were usually totally ruthless with their enemies) were known to stand at attention on the bridges of their warships and salute the sinking American, Dutch, and British Commonwealth vessels which had fought bravely against them. They were honoring the courage of the sailors who fought them. (this did not mean, however, that they spent much attention on rescuing those men from the water or treating them decently as prisoners...prisoners were thought to have lost their honor in the Japanese credo, and were therefore treated very badly in most cases).